System and method for estimating the position and orientation of a mobile communications device in a beacon-based positioning system

ABSTRACT

An example of a lighting device including a light source, a modulator and a processor. The processor is configured to control the light source to emit light for general illumination and control the modulator to modulate the intensity of the emitted light to superimpose at least two sinusoids. Frequencies of the at least two sinusoids enable a mobile device to infer the physical location of the lighting device.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/823,074 filed Aug. 11, 2015, entitled “SYSTEM AND METHOD FORESTIMATING THE POSITION AND ORIENTATION OF A MOBILE COMMUNICATIONSDEVICE IN A BEACON-BASED POSITIONING SYSTEM,” the disclosure of whichalso is entirely incorporated herein by reference.

U.S. patent application Ser. No. 14/823,074 claims the benefit of U.S.Provisional Application No. 62/036,254 filed Aug. 12, 2014, entitled“SYSTEM AND METHOD FOR ESTIMATING THE POSITION AND ORIENTATION OF AMOBILE COMMUNICATIONS DEVICE IN A BEACON-BASED POSITIONING SYSTEM,” thedisclosure of which also is entirely incorporated herein by reference.

FIELD OF THE INVENTION

This disclosure relates generally to a system and method for estimatingthe position and orientation of a mobile device with respect to alight-based positioning system.

BACKGROUND

Described herein are techniques for estimating the position andorientation of a light-detecting mobile communications device (e.g.,cellular telephone, tablet computer, wearable computing device,electronically enhanced eyeglasses) by identifying light beacons in thevicinity of the mobile device and by compensating at least in part formotion of the mobile device, the presence of visual noise, and localirregularities in the Earth's magnetic field.

Indoor positioning services relate to methods in which networks ofdevices and algorithms are used to locate mobile devices withinbuildings. Indoor positioning is regarded as a key component oflocation-aware mobile computing and is a critical element in providingaugmented reality (AR) services. Location-aware computing relates toapplications that utilize a mobile device user's location to providecontent relevant to that location. Additionally, AR is a technology thatoverlays a virtual space onto a real (physical) space. To successfullyenable AR and location-aware computing, accurate indoor positioning is akey requirement. Moreover, indoor positioning and AR services mayinclude displaying on a user's mobile device in real time a spatial mapwhich includes “you are here” information; such information not onlyshould be accurate enough to assist user navigation (e.g., in a retailspace) but should be presented in a manner that is clear and agreeableto the user.

Signals from Global Positioning System (GPS) satellites lose significantpower when passing through construction materials, and suffer frommulti-path propagation effects that make GPS unsuitable for indoorenvironments. Techniques based on received signal strength indication(RSSI) from WiFi and Bluetooth wireless access points have also beenexplored for indoor positioning. However, complex indoor environmentscause radio waves to propagate in dynamic and unpredictable ways,limiting the accuracy of positioning systems based on RSSI. Ultrasonictechniques, which transmit acoustic waves to microphones, can also beused to approximate indoor position. However, ultrasonic sound wavesoperate at lower frequencies than systems based on WiFi and attenuatesignificantly when passing through walls. This attenuation, which limitsthe spatial reach of waves from an ultrasound source, potentially makesultrasonic techniques more accurate than WiFi or Bluetooth techniques.

Optical indoor positioning techniques use optical signals, eithervisible or infrared, and can be used to accurately locate mobile devicesindoors. These are more accurate than the approaches mentionedpreviously, since optical signals are highly directional and cannotpenetrate solid objects. However, several limitations, drawbacks, orpotential sources of error in optical indoor positioning techniques mayneed to be addressed.

These include, firstly, a need to reduce noise in the signal derived bya mobile device from images or ambient light levels. Any scheme todetect a signal mixed with noise is made more reliable by reduction ofthe noise. In particular, an illustrative light-source detection schemedescribed herein, according to various embodiments of the invention,depends on the detection of spectral peaks (i.e., peaks in the frequencydomain) that correspond to identification signals emitted by lightsources. The spectrum of a digital image (or other data obtained bysensing light, whether using an image-forming camera, anon-image-forming sensor, or both) is estimated by calculating a FastFourier Transform (FFT) of the image or a signal derived by averagingfrom the data. Each light source emits light having an at least locallyunique spectrum whose distinct features (e.g., peaks) constitute theidentification code (ID) of that light. ID detection depends on theidentification of patterns of peaks that may be obscured or renderedambiguous by noise in the signal. In essence, signal-to-noise ratio mustexceed some threshold for detection to be possible.

A second limitation of indoor positioning that may be addressed is thepresentation of location information in a user-friendly way. In abeacon-based positioning system that may show a user of a mobile devicetheir approximate position and orientation on a map displayed on themobile device, sudden movement of the user's position indicator from onepoint to another (e.g., from one beacon location to another beaconlocation, or to the centroid of two or more beacon locations) tends tobe disconcerting or irksome to the user. It is therefore desirable toform an estimate of a user's position that moves smoothly, or nearly so,between points on a map.

Thirdly, it is desirable that usefully accurate orientation informationbe delivered to users of an indoor position system, including thebearers of mobile devices who may be viewing maps of their spatialcontext on their device displays. Many mobile devices contain a compassor magnetometer that provides heading or orientation information bysensing the Earth's magnetic field. However, in portions of many indoorspaces, the Earth's magnetic field may, in effect, be locally distortedby the proximity of masses of metal or devices that generate magneticfields. In such areas, raw device measurements of orientation may bemisleading. It is therefore desirable to assure that a user's map isaccurately oriented.

BRIEF DESCRIPTION OF THE FIGURES

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a representation of a mobile device receiving light from a LEDlight source.

FIG. 2 is a representation of a mobile device receiving multiple sourcesof light simultaneously from multiple LED light sources.

FIG. 3 is a representation of the internal components commonly found ina LED light source that is capable of being modulated to send digitaldata.

FIG. 4 illustrates information which can be optically transmitted froman LED light source.

FIG. 5 is a representation of the components which are commonly found inmobile devices which enable them to receive optical signals from LEDsources.

FIG. 6 is a representation of multiple LED light sources sending uniqueinformation to multiple mobile devices.

FIG. 7 illustrates the process of a mobile device sending identificationinformation and receiving location information via a network to aserver.

FIG. 8 illustrates the high level contents of the server which includesdatabases and web services for individual areas enabled with lightpositioning systems.

FIG. 9 illustrates the components inside the databases.

FIG. 10 illustrates the information contained in the Light IDs database.

FIG. 11 illustrates the information contained in the Maps database.

FIG. 12 illustrates the information contained in the Content database.

FIG. 13 illustrates the information contained in the Analytics database.

FIG. 14 illustrates the process of a mobile device receiving locationand content information via a light-based positioning system.

FIG. 15 is a process illustrating the background services and how theyactivate various sensors contained inside the mobile device.

FIG. 16 illustrates the process of combining multiple informationsources with a light-based positioning service.

FIG. 17 illustrates how a client accesses multiple light positioningenabled locations with multiple mobile devices.

FIGS. 18A-C are representations of a light source undergoingpulse-width-modulation at varying duty cycles, according to someembodiments of the present disclosure.

FIGS. 19A-C are representations of a light source undergoingpulse-width-modulation at varying duty cycles with a DC offset,according to some embodiments of the present disclosure.

FIG. 20 is a block diagram of a “Digital Pulse Recognition” (DPR)modulator with a dimming control system for a light source, according tosome embodiments of the present disclosure.

FIG. 21 is a representation of a block diagram of a DPR modulator,according to some embodiments of the present disclosure.

FIG. 22 is a block diagram of an encoder for DPR modulation, accordingto some embodiments of the present disclosure.

FIG. 23 is a block diagram for a waveform generator for DPR modulation,according to some embodiments of the present disclosure.

FIG. 24 is a block diagram of a symbol selector system module, which isused to select an appropriate symbol for use in DPR modulation,according to some embodiments of the present disclosure.

FIG. 25 is a plot of a camera sampling function, according to someembodiments of the present disclosure.

FIG. 26 is a plot of a modulated illumination function undergoing DPRmodulation at a frequency of 300 Hz, according to some embodiments ofthe present disclosure.

FIG. 27 is a plot of a convolution of a camera sampling function and aDPR modulated light signal, according to some embodiments of the presentdisclosure.

FIG. 28 is a model of the CMOS sampling function for a rolling shutter,according to some embodiments of the present disclosure.

FIG. 29 is a plot of a sampling function for a CMOS rolling shutter overmultiple frames, according to some embodiments of the presentdisclosure.

FIG. 30 is a high level flow chart of an algorithm for configuring amobile device to receive DPR modulated signals, according to someembodiments of the present disclosure.

FIG. 31 is a high level flow chart of an algorithm for minimizing andlocking camera settings using existing mobile device applicationprogramming interfaces (APIs), according to some embodiments of thepresent disclosure.

FIG. 32 is a high level flow chart of an algorithm for receiving DPRsignals on an image sensor, according to some embodiments of the presentdisclosure.

FIG. 33 is a high level flow chart of an algorithm for determining tonesembedded within a DPR illuminated area, according to some embodiments ofthe present disclosure.

FIG. 34 is a high level flow chart of an algorithm for performingbackground subtraction on images gathered from a DPR illuminated scene,according to some embodiments of the present disclosure.

FIG. 35 is a high level flow chart of an algorithm for performing motioncompensation on video frames when performing DPR demodulation, accordingto some embodiments of the present disclosure.

FIG. 36 is a photograph of a surface under illumination from DPRmodulated signals, according to some embodiments of the presentdisclosure.

FIG. 37 is a post-processed image of a DPR modulated scene afterperforming background subtraction, according to some embodiments of thepresent disclosure.

FIG. 38 is a post-processed image of a DPR modulated scene after rowaveraging, according to some embodiments of the present disclosure.

FIG. 39 is a plot of the 1-D spectral content of a DPR modulatedsurface, according to some embodiments of the present disclosure.

FIG. 40 is a plot of the 1-D spectral content of a DPR modulated surfaceafter removing DC bias, according to some embodiments of the presentdisclosure.

FIG. 41 is a 2-D FFT of a DPR modulated surface, according to someembodiments of the present disclosure.

FIG. 42 is a 2-D FFT of a DPR modulated surface after applying a lowpass filter, according to some embodiments of the present disclosure.

FIG. 43 is a 2-D FFT of a DPR modulated surface after applying a highpass filter, according to some embodiments of the present disclosure.

FIG. 44A is an in-focus image of a portion of a typical visualbackground containing periodic noise.

FIG. 44B is an out-of-focus image of the scene of FIG. 44A.

FIG. 45 is a plot comparing the FFT of the image of FIG. 44A to the FFTof the image of FIG. 44B.

FIG. 46 is a high level flow chart of a method for updating a mobiledevice position estimate according to an embodiment of the invention.

FIG. 47 is a high level flow chart of another method for updating amobile device position estimate according to an embodiment of theinvention.

FIG. 48 depicts the motion of a mobile device in a physical space andthe representation of that motion according to two methods of estimatingthe device's position.

FIG. 49 depicts the relationship of a mobile device to the Earthmagnetic field and to a locally perturbed magnetic field.

FIG. 50A depicts the relationship of the Earth magnetic field to aperturbation of that field.

FIG. 50B depicts the relationship of the Earth magnetic field to adifferent perturbation of that field.

FIG. 51A depicts an incorrect map orientation on a mobile device causedby a local perturbation of the Earth magnetic field.

FIG. 51B depicts a correct map orientation on a mobile device aftercompensation a local perturbation of the Earth magnetic field.

FIG. 52 is a high level schematic of a method for correcting the headingcalculated by a mobile device in the presence of a perturbed Earthmagnetic field.

FIG. 53 is a high level schematic of a method for calibrating a systemfor correcting the compass headings measured by a mobile device.

FIG. 54 is a high level schematic of a method for continuouslyrecalibrating a system for correcting the compass headings measured by amobile device.

FIG. 55A depicts the relationship of sinusoidal brightness signal to arolling shutter exposure interval.

FIG. 55B depicts the relationship of sinusoidal brightness signal toanother rolling shutter exposure interval.

FIG. 56A depicts the contribution of a sinusoidal brightness signal torows of pixels in a rolling shutter image when the exposure intervalequals the period of the sinusoid.

FIG. 56B depicts the contribution of a sinusoidal brightness signal torows of pixels in a rolling shutter image when the exposure intervalonly approximately equals the period of the sinusoid.

FIG. 56C depicts the contribution of a sinusoidal brightness signal torows of pixels in a rolling shutter image when the exposure interval isdistinct from the period of the sinusoid.

FIG. 57A depicts the amplitude of a sinusoidal brightness signal,frequency 675 Hz, detected in a rolling shutter image for a range ofexposure durations.

FIG. 57B depicts the amplitude of a sinusoidal brightness signal,frequency 644 Hz, detected in a rolling shutter image for a range ofexposure durations.

FIG. 57C depicts the amplitude of a sinusoidal brightness signal,frequency 704 Hz, detected in a rolling shutter image for a range ofexposure durations.

FIG. 57D depicts the amplitude of a frequency-swept brightness signal,center frequency 675 Hz, detected in a rolling shutter image for a rangeof exposure durations.

FIG. 58A depicts the alignment of a rectangular physical spacecontaining a light source with a Cartesian coordinate system.

FIG. 58B depicts the encoding of the location of the light source inFIG. 58A using a first position-encoding method according to variousembodiments of the invention.

FIG. 58C depicts the encoding of the location of the light source inFIG. 58A using a second position-encoding method according to variousembodiments of the invention.

FIG. 59 depicts frequency-domain relationships using a thirdposition-encoding method according to various embodiments of theinvention.

FIG. 60 depicts encoding of the location of the light source in FIG. 58Ausing the third position-encoding method according to variousembodiments of the invention.

FIG. 61A depicts the location of a second light source in the spacedepicted in FIG. 58A.

FIG. 61B depicts encoding of the location of the light source in FIG.61A using the third position-encoding method according to variousembodiments of the invention.

FIG. 62 is a high level flow chart of an example of a process performedby a mobile device to determine a location of a lighting device within aspace.

DETAILED DESCRIPTION

The present disclosure relates, for example, to a method for frequentlyupdating an estimate of a mobile device's position with respect to oneor more beacon light sources. The method, in some examples, updates adevice position estimate (e.g., a two-dimensional position estimate) asa sum of weighted position vectors derived from detections by the deviceof beacons having previously determined locations. A light-sensingapparatus (e.g., forward-facing camera, rear-facing camera, and/or otherlight-sensing device comprised by the mobile device) of the mobiledevice is employed, in various examples, to acquire digital images (ornon-image data) at a certain frame rate; the images are processed in amanner described herein in order to detect the presence of one or morebeacon light sources; and an estimate of the mobile device's position ismodified (updated) based on the one or more beacon light sourcesdetected. Updates to the position estimate may be made at a rate limitedby the image frame acquisition rate of the mobile device. The positionestimate changes discretely both in time (i.e., upon beacon detection inimage frames) and in space (by vector increments based on beacondetections), but in general, the position estimate will be perceived bya user as changing smoothly or nearly so. Herein, “camera” is to beconstrued broadly as referring, as appropriate, not only toimage-sensing devices but to all optical sensors capable of acquiringdata that contain light-encoded information. Also, herein “image” is tobe construed broadly as referring, as appropriate, to any data set,however obtained, that may contain light-encoded information.

Various examples employ noise reduction techniques in images to enablemore sensitive and accurate detection of beacon light sources as a basisfor position-estimate updating. Such techniques include, but are notlimited to, (a) background subtraction employing multiple images of asingle scene or portions thereof and (b) deliberate defocusing of imagesto mitigate the potentially confounding presence of regular patterns.Defocusing effectively increases signal-to-noise ratio in variousembodiments of the present invention.

Other various examples enable correction of a mobile device's sensedorientation (e.g., compass heading) by information contained in aserver. The correction to be applied may vary with the estimatedlocation of the mobile device (e.g., in different parts of a retailspace) and with time, as local deviations from the earth magnetic fieldmay change when equipment, wiring, and the like are installed orrepositioned. A local correction to be applied may also be specific tothe particular model of mobile device in question, as different devicemodels may experience different deviation errors even under identicalenvironmental conditions. Various examples employ adaptive,crowd-sourced data collection from one or more mobile devices to updatethe corrections to be applied to the compass headings of one or moremodels of mobile device.

In another example, the frequencies of signals transmitted by lightsources are “swept” or varied through time, either continuously or indiscrete increments, in order to assure robust detection of the signalsregardless of the exposure parameters independently selected by a mobiledevice (e.g., exposure time of a photograph of video frame).

In another example, the physical coordinates of a light source areencoded by modulating the output of the light source. Such encoding maybe achieved by a variety of methods in various embodiments: such methodsinclude, but are not limited to, (a) the simultaneous modulation oflight-source brightness by two sinusoids of different frequency andamplitude in a single frequency band, (b) the simultaneous modulation oflight-source brightness by two sinusoids of different frequency andamplitude in two nonoverlapping frequency bands, or (c) the simultaneousmodulation of light-source brightness by three sinusoids of differentfrequency in a single frequency band. The frequencies of such positionalinformation signals may be “swept” or varied through time, eithercontinuously or in discrete increments, in order to mitigate the effectsof destructive interference by multiple lights illuminating overlappingareas and so facilitate robust detection of the signals. The durationand other aspects of such sweeping may be varied in a random orpseudorandom manner in order to minimize or substantially eliminatedestructive interference at any point in the working space of thesystem.

Thus, various examples provide for the frequent, robust, and adaptiveupdating of both absolute position information and orientationinformation for mobile devices in a beacon-based positioning system. Itis among the advantages realized by the invention that a user of amobile device in the operating space will, in general, be offered a moretimely, easily-observed estimate of their location and a more accurateestimate of their device orientation than would offered usingconventional techniques.

Systems and methods are provided that disclose providing a positioningservice for devices based on light received from one or more lightsources. This light-based positioning service uses light informationtransmitted by each light source to determine the position of thedevice. The device captures the one or more light sources and is thenable to detect the information transmitted by each of the light sources.The light information may include an identification code that is used toidentify the position of the light source. By capturing more than onelight source on the device the accuracy of the device's position may beimproved. The position information may then be used to provide relevantcontent information to the user. The light sources are each independentbeacons that transmit individual identification information throughlight.

In some embodiments light sources are used to provide an indoorpositioning service to mobile devices. Each light source is given anidentification code, corresponding to an associated database, whichcontains information that ties the light source to specific locationdata. The identification codes are broadcasted through visible light bymodulating the LED light source. The modulation occurs at speeds thatare undetectable by the human eye, yet appropriate to be received by acamera equipped mobile device. The mobile device receives theidentification information, and uses it to lookup its indoor position inthe form of location data. Since the identification information istransmitted through visible light, which is highly directional, themobile device is known to be within the line of sight of the LED lightsource. Since the indoor position of the LED light source is known frombuilding floor plans and lighting plans, the corresponding indoorposition of the mobile device can be determined.

Another embodiment describes a scenario where a mobile device is in viewof three or more LED light sources. Each source emits uniqueidentification information and, with knowledge of the relative positionsof each LED light source, one can calculate the device's relativeposition in three dimensions. This process utilizes photogrammetricimage processing techniques to identify and calculate coordinates forthe positions of the light sources in order to relatively locate themobile device.

Yet another embodiment includes a system by which a mobile device 103may receive content based upon identification information received fromeither one or more LED light sources. The identification information isused to access a database that correlates LED lights and content. Anexample of such a use case is a mobile device user in a museum, whoreceives identification information from a light source illuminating anexhibit, and then uses the received identification information to obtainadditional content about the exhibit.

FIG. 1 represents a mobile device 103 receiving light 102 from a LEDlight source 101. The LED light source 101 may be any lighting sourceused for general purpose, spot illumination, or backlighting. The LEDlight source may come in several form factors but is not limited to:Edison screw in, tube style, large and small object backlighting, oraccent lighting spots and strips. For the purposes of this disclosure,any form of LED light is considered as a potential source capable oftransmitting information.

Light 102 is a modulated LED light source 101, and is part of thevisible electromagnetic wireless spectrum. LEDs are considered digitaldevices which may be rapidly switched on and off, to send signals abovethe rate that the human eye can see. This allows them to be exploited tosend digital data through the visible light itself. By modulating theLEDs, turning them on and off rapidly, one may send digital informationthat is unperceivable to the human eye, but is perceivable by applicablesensors, including but not limited to image sensors and other types ofphotosensors.

There are many modulation techniques used to send information throughlight 102. One technique, “On Off Keying” (OOK), is a scheme to transmitdigital data by rapidly switching a signal source on and off. OOK is thesimplest form of amplitude-shift keying (ASK) which is a modulationtechnique that represents digital data through either the presence orabsence of a carrier wave. When communicating with visible light, thecarrier wave takes the form of the transmitted light signal. Thereforeat a rudimentary level, when the light signal is turned “on” a digital“one” is perceived, and when the light signal is turned “off” a “zero”is perceived. Furthermore the rate at which the light signal is turnedon and off represents the modulation frequency. Note that regardless ofchanging the modulation frequency, the “carrier wave” remains unchangedas this is an inherent property of the light itself. For example thecarrier wave corresponding to a blue light signal is uniquely differentthan the carrier wave corresponding to a red light signal. While thesetwo signals differ only in the wavelength specific to their perceivedcolor, they can be perceived as two discrete signals.

In addition to OOK, another possible technique is defined as “DigitalPulse Recognition” (DPR). This modulation technique exploits the rollingshutter mechanism of a complementary metal-oxide-semiconductor (CMOS)image sensor. Due to their superior energy efficiency, CMOS sensors arepreferred to charge-coupled device (CCD) sensors on mobile devices. Whena CMOS image sensor with a rolling shutter takes an image, it does notexpose the entire image simultaneously. Instead, the rolling shutterpartially exposes different portions of the frame at different points intime. Typically, this causes various unwanted effects: skew, wobble, andpartial exposure. In the presence of an LED light driven by a pulsewidth modulated signal, images received from a CMOS sensor exhibit“residual banding” in the form of visible distortions. The image appearsto have alternating dark/white stripes. The stripes are a direct resultof the rolling shutter mechanism, and their width is proportional to thefrequency of the pulse width modulated (PWM) signal. Higher frequenciescorrespond to narrower stripes, and lower frequencies result in widerstripes. Practical frequency ranges for use with this technique arebetween 60 Hz and 5000 Hz. This technique allows one to exploit therolling shutter mechanism to recover digital data from an opticallyencoded signal.

DPR has the potential for much higher data rates than both OOK andfrequency shift keying (FSK). In FSK and OOK, the camera's frame ratelimits the data rate. The highest possible data rate is half of theframe rate, since each symbol spans over two frames. In DPR modulation,a single frame is sufficient for capturing the transmitted symbol.Furthermore, symbols are not “binary” —there are can be as many as 30different possibilities for a symbol.

In the DPR modulation scheme, image processing is used to measure thestripe width of the recorded image. By successively changing the LEDdriver frequency for each frame, information is essentially transmittedthrough recognition of the band widths. In the current design, 10separate frequencies are used. For a 30 frames per second (FPS) camera,this corresponded to an effective data transfer rate of ˜100 bits persecond (bps).

Both of these techniques are interesting because they can allow thetransmission of information through single color light sources, insteadof having to create lighting sources which contain multiple colorlights. In the world of LED lighting products, white light is majorlyachieved by layering a phosphorous coating on top of blue LEDs. Thecoating creates the visible perception of “white” light, instead ofblue. The alternative to this can be achieved through combining red,green, and blue LED lights; however this approach is expensive and powerinefficient as the lumens per watt properties differ between differentcolored LEDs. Blue LEDs are generally more energy efficient than theirred and green counterparts, which is why they are used in mostcommercial LED lighting products. It is because of this reason that itmakes the most sense to use a data modulation technique that uses asingle wavelength of light, rather than multiple, because this complieswith LED lighting products.

In addition to LED light sources, other types of light sources are alsocapable of transmitting information through modulation. Alternativeincandescent and fluorescent technologies can also be exploited toachieve data transmission, however the circuitry is more complex becausethe turn-on and turn-off times of incandescent and fluorescent lightsare subject to additional factors.

The modulation frequency of the light source is highly dependent on thereceiving circuitry. While incandescent and fluorescent technologiesgenerally do not “flicker” on and off during the course of normaloperation, LED lighting sources are sometimes designed to flicker abovethe rate which the eye can see in order to increase their longevity, andconsume less power. Most humans cannot see flicker above 60 Hz, but inrare instances can perceive flicker at 100 Hz to 110 Hz. To combat this,lighting manufacturers design flicker above 200 Hz into their lightingproducts.

Mobile device 103 may be a smart mobile device and is most commonlyfound in the form of mobile phones, tablets, and portable laptopcomputers. In order for a mobile device 103 to receive information 102from the LED light source 101 it has an embedded or attached sensorwhich is used to receive the incoming light 102 signals. One such sensoris a camera, which has a typical frame refresh rate between fifteen andsixty frames per second (fps). The fps is directly related to the speedat which optical signals can be transmitted and received by the camera.The sensor may capture a number of successive image frames that maylater be analyzed to determine if a light source is providinginformation through light.

Mobile device 103 may include a processor, module, memory, and sensor inorder to capture and analyze light received from light sources. Themobile device may analyze the successive image frames captured by thesensor by using the module. The module may be logic implemented in anycombination of hardware and software. The logic may be stored in memoryand run by processor to modify the successive images and analyze thesuccessive images to determine information encoded in the light of oneor more light sources. The module may be built in to the mobile deviceto provide the capabilities or it may be downloaded and installed. Themodule may be an application that runs on the mobile device whenselected by a user. The module may also be used to receive content andother information related to the position of the mobile device and toprovide this content to other modules or to the mobile device.

The reception of optically transmitted information is particularlyinteresting when used as an indoor positioning system. In a light-basedpositioning system, the physical locations of light sources may be usedto approximate the relative position of a mobile device 103 within lineof sight. On the mobile side, in addition to a receiving module, themobile device 103 may use information to determine position of themobile device. The mobile device may access a data source containinginformation about where the lights are physically located to determineposition. This data source may be stored locally, or in the case wherethe mobile device 103 has a network connection, the data source may bestored on an external server 703.

For scenarios where a network connection is not available, beforeentering an indoor space the mobile device 103 may optionally download a“map pack” containing the information used to locate itself indoors,instead of relying on an external server 703. In order to automate thisprocess, the mobile device 103 would first use an alternative existingtechnique for resolving its position and would use the gained locationinformation to download the appropriate map pack. The techniques forreceiving geo-location information include, for example, GPS, GSM, WiFi,user input, accelerometer, gyroscope, digital compass, barometer,Bluetooth, and cellular tower identification information. Thesetechniques may also be used to fill gaps between when a position of themobile device is determined using the light-based technique. Forexample, a mobile device may be placed at times so its camera does notcapture light sources. Between these times these alternative existingtechniques may be used for filling in position and location informationthat may be helpful to the user. The map pack would contain a map 902 ofthe indoor space the user is entering, locations of the lights from somesort of existing or third-party lighting plan 1103, and anylocation-dependent content 903 for the mobile device 103 to consume. Anyrequests for location information would simply access data storedlocally on the mobile device 103, and would not need to access a remoteserver via a network 601.

In terms of the experience when using a light-based positioning system,the indoor location reception and calculation may happen with little tono user input. The process operates as a background service, and readsfrom the receiving module without actually writing them to the displayscreen of the mobile device. This is analogous to the way WiFipositioning operates, signals are read in a background service withoutrequiring user interaction. The results of the received information maybe displayed in a number of ways, depending on the desired application.In the case of an indoor navigation application, the user would see anidentifying marker overlaid on a map of the indoor space they are movingaround in. In the case of content delivery, the user might see a mobilemedia, images, text, videos, or recorded audio, about the objects theyare standing in front of.

In scenarios where the mobile device 103 is in view of several lightsources, it may receive multiple signals at once. FIG. 2 is arepresentation of a mobile device 103 receiving identificationinformation 102 a-102 c from multiple LED light sources 101 a-101 c.Each light source is transmitting its own unique piece of information.In order to identify its position or receive location-based content, themobile device 103 may then use the received information to access adatabase 802 containing information about the relative positions of theLED light sources 101 a-101 c and any additional content 903. When threeor more sources of light are in view, relative indoor position may bedetermined in three dimensions. The position accuracy decreases withless than three sources of light, yet remains constant with three ormore sources. With the relative positions of lights 101 a-101 c known,the mobile device 103 may use photogrammetry to calculate its position,relative to the light sources.

Photogrammetry is a technique used to determine the geometric propertiesof objects found in photographic images. In the context of locatingmobile devices using light sources, photogrammetry refers to utilizingthe corresponding positions of LED light sources, and their positions in3-D space, to determine the relative position of a camera equippedmobile device. When three unique sources of light are seen by the cameraon a mobile device, three unique coordinates may be created from thevarious unique combinations of 101 a-101 c and their relative positionsin space can be determined.

For a mobile device 103 equipped with an image sensor the followingscenario may be considered. When multiple LED light sources appear inthe image sensors field of view, the sources appear brighter relative tothe other pixels on the image. Thresholds may then be applied to theimage to isolate the light sources. For example, pixel regions above thethreshold are set to the highest possible pixel value, and the pixelregions below the threshold are set to the minimum possible pixel value.This allows for additional image processing to be performed on theisolated light sources. The end result is a binary image containingwhite continuous “blobs” where LED light sources are detected, and darkelsewhere where the sources are not detected.

A blob detection algorithm may then be used to find separate LED lightsources. A minimum of three separate LED blobs are used to resolve the3-D position of a mobile device 103. Each LED blob represents a “regionof interest” for the information reception, and is simultaneouslytransmitting a unique piece of information via the modulated visiblesignal from the light source. For the purposes of reception, each regionof interest is processed independently of other regions of interest andis considered to be uniquely identifiable. A center of mass calculationfor each region may be performed to determine the pixel coordinates ofthe center of each LED light source. This center of mass calculation isperformed for each frame to track the regions of interest as they movearound the image.

Once the regions of interest are established, a detection algorithmcaptures multiple image frames for each region of interest in order toreceive the visible light signal contained in each blob. For each framein a detected region of interest, a threshold algorithm determineswhether the frame contains a “1” (in the case of an aggregate pixelvalue above the threshold), or a “0” (in the case of an aggregate pixelvalue lower than the threshold). The threshold algorithm is used sincethe communication is asynchronous, so the camera receiver period mayoverlap between the transmission of a “1” and a “0” from the LED lightsource.

The result of converting successive image frames in a region of interestto binary values is in essence a down-sampled digital version of thesignal received from the LED light source. Next, demodulation of thedown-sampled digital signal is used to recover the transmitted bits.This down sampling is used due to the fact that the signal modulationfrequency should be above the rate at which the human eye can see, andthe image sensor frame rate is typically limited to 15-30 fps.

At a lower level, the mobile device 103 processes data on aframe-by-frame basis. Each frame is split into separate regions ofinterest, based on the detection of light sources. For each region ofinterest, a thresholding algorithm is used to determine whether a givenregion is “on” or “off”. This is done by taking the average pixel valuefor the region and comparing it to the threshold value. If the region is“on”, the demodulator assumes the light source has just transmitted a“1”. If the region is “off”, the demodulator assumes the light sourcehas sent a “0”. The result of this is the equivalent of a 1-bitanalog-to-digital conversion (ADC), at a sampling rate which is equal tothe frame rate of the camera.

After a frame is processed, the results of the ADC conversation arestored in a circular buffer. A sliding correlator is applied to thebuffer to look for the presence of start bits 402. If start bits 402 arefound, the demodulation algorithm assumes it is reading a valid packetof information 401 and proceeds to capture the rest of the transmission.Two samples are used for each bit, so the algorithm creates a linearbuffer that is twice the size of the remaining packet. Each subsequentADC is written sequentially to the linear buffer. When the linear bufferis filled, the demodulation algorithm performs a Fast Fourier Transform(FFT) on the buffer to recover the transmitted signal.

FIG. 3 describes internal components commonly found in LED light source101 with the addition components to allow for the transmission ofoptical signals. The LED light source 101 typically contains analternating current (AC) electrical connection 301 where it connects toan external power source, an alternating current to direct current(AC/DC) converter 302 which converts the AC signal from the power sourceinto an appropriate DC signal, a modulator 304 which interrupts power tothe LEDs in order to turn them on and off, a microcontroller 305 whichcontrols the rate at which the LEDs are modulated, and a LED drivercircuit 303 which provides the appropriate amount of voltage and currentto the LEDs.

Electrical connection 301 is an electrical source that is used to supplypower to the LED light source 101. This most commonly comes in the formof a 120 Volt 60 Hz signal in the United States, and 230 Volt 50 Hz inEurope. While depicted in FIG. 3 as a three pronged outlet, it may alsotake the form of a two terminal Edison socket which the bulb is screwedinto, or a bundle of wires containing a live, neutral, and/or ground.When considering other forms of lighting such as backlighting and accentlighting, the electrical connection may also come in the form of a DCsource instead of an AC source.

Most LED light sources contain an AC/DC converter 302 that converts thealternating current from the power source 301 to a direct current sourceused internally by the components found inside the bulb or light source.The converter takes the alternating current source commonly found inexisting lighting wiring and converts it to a direct current source. LEDlight sources generally use direct current, therefore an AC/DC converteris found in most lighting products regardless of form factor.

LED driver 303 provides the correct amount of current and voltage to theLEDs contained inside the lighting source. This component is commonlyavailable and may have either a constant-current or constant-voltageoutput. The LEDs found inside most lighting sources arecurrent-controlled devices, which require a specific amount of currentin order to operate as designed. This is important for commerciallighting products because LEDs change color and luminosity in regards todifferent currents. In order to compensate for this, the LED drivercircuitry is designed to emit a constant amount of current while varyingthe voltage to appropriately compensate for the voltage drops acrosseach LED. Alternatively, there are some high voltage LEDs which requirea constant voltage to maintain their color and luminosity. For thesecases the LED driver circuitry provides a constant voltage while varyingthe current.

Modulator 304 serves the function of modulating the LED light source 101on and off to optically send light 102 signals. The circuits featuringthe modulator may simply consist essentially of solid-state transistorscontrolled by a digital input. In essence, the modulator 304 turns theLEDs on and off by allowing or preventing current flow. When currentflows through the modulator with the switches closed the LEDs turn on,and when the switches are open in the modulator no current can flow andthe LEDs turn off. When the modulator is controlled by an additionallogic component, it has the ability to send repeating patterns of on/offsignals in order to transmit digital data through the visible light 102.The modulator interfaces directly in between the AC/DC converter 302 andthe LED driver 303, and is controlled by a microcontroller 305.

The microcontroller 305 provides the digital input signal to themodulator unit 304. This function may also be achieved using afield-programmable gate array (FPGA), but typically consumes more powerwith added complexity. The microcontroller's 305 task is to send apre-determined sequence of signals to the modulator 304 which theninterfaces with the LED driver 303 to modulate the outgoing visiblelight from the LED source 101. The microcontroller contains anonvolatile memory storage area, which stores the identification code ofthe light signal. Examples of possible nonvolatile memory sourcesinclude programmable read only memory (PROM), electrically erasableprogrammable read only memory (EEPROM), or Flash.

In regards to the microcontroller pins, the microcontroller 305 containsa digital output pin, which is used to modulate the light output. Togenerate the output signal waveforms, timer modules within themicrocontroller 305 are used. Typical logic levels for the digitaloutput are 3.3V and 5V. This digital output feeds into the modulator 304which interrupts the driver circuit 303 for the LED light source 101.Alternatively, if the LED light source requires lower power, such asbacklighting or individual LED diodes, the output of the microcontroller305 could also be used to drive the light sources directly.

The sequence of signals sent from the microcontroller 305 determines theinformation that is transmitted from the LED light source 101. FIG. 4describes the information 401 format of the optically transmittedinformation from the light 102. At the highest level, each packet ofinformation contains some sort of starting bit sequence, which indicatesthe beginning of a packet, followed by data 403, and some sort of errordetection identifier. The size and position of each portion ofinformation is dependent on the application and is also constrained byrequirements of the receiving device.

Each packet of information 401 transmitted from the LED light source 101contains a sequence of starting bits 402, followed by data 403, and thenterminated with an error detection code 404. Since the LED light sources101 are continually broadcasting information 401, erroneous packets aresimply discarded while the receiver listens for the starting bits 402,indicating the beginning of the next packet. In cases where multiplesources of light are observed by a mobile device 103, multiple pieces ofinformation 401 are received simultaneously.

Information 401 describes the encoded information that is transmitted bythe LED light source 101. The information 401 is contained in a packetstructure with multiple bits which correspond to numeric integer values.The data 403 portion of the information packet may include unique IDcodes 701. Currently the data 403 size is set to 10 bits, but may be ofvarying length. Each bit represents a binary “1” or “0”, with 10 bits ofdata 403 corresponding to 1024 possible values. This corresponds to 1024unique possibilities of ID codes 701 before there is a duplicate. The IDcode may include location information in the ID code that provides ageneral indication of geographical location of the light. Thisgeographical location information may be used to more quickly locatelight source information that is used in determining indoor positioningon the mobile device. For example, the geographical information maypoint to a database to begin searching to find relevant information forpositioning. The geographical information may include existing locationidentifiers such as area code, zip code, census tract, or any othercustomized information.

The ID code 701 is static and is assigned during the calibration phaseof the LED light source 101 during the manufacturing process. One methodto assign the ID code 701 is to place instructions to generate a randomcode in the nonvolatile memory. Once the LED light source 101 is poweredon the microcontroller reads the ID code 701 from the nonvolatile memorystorage area, and then uses this code for broadcasting each and everytime it is subsequently powered on. Since the ID code 701 is static,once it is assigned it will be forever associated locally to thespecific LED light source 101 which contains the microcontroller 305.

FIG. 5 describes the components found in mobile devices 103 that arecapable of receiving optical information. At the highest level themobile device contains an image sensor 501 to capture opticallytransmitted information, a central processing unit 502 to decipher andmanage received information, and a network adapter 503 to send andreceive information.

Photosensors are devices which receive incoming electromagnetic signals,such as light 102, and convert them to electrical signals. In a similarfashion, image sensors are arrays of photosensors that convert opticalimages into electronic signals. The ability to receive signals frommultiple sources is an important benefit when using image sensors forreceiving multiple optical signals.

Image sensor 501 is a typical sensor which is found in most smartdevices. The image sensor converts the incoming optical signal into anelectronic signal. Many devices contain complementarymetal-oxide-semiconductor (CMOS) image sensors; however, some still usecharge-coupled devices (CCD). CMOS image sensors are the more popularchoice for mobile devices due to lower manufacturing costs and lowerpower consumption. There are several tradeoffs to consider when choosingan image sensor to perform photogrammetry on multiple LED light sources101. One tradeoff is between the camera resolution and the accuracy ofthe photogrammetric process when triangulating between multiple lightsources—increasing the number of pixels will increase the accuracy.There is also another tradeoff between the data rate of the transmissionand the sampling rate (in frames per second) of the camera. The datarate (in bits/second) is half the frame rate of the camera (e.g., a 30fps camera will receive 15 bps). And finally when determining the lengthof the information 401 packet, the larger the size the longer thereception period, as more bits generally requires longer samplingperiods to capture the full message.

CPU 502 is typically a generic CPU block found in most smart devices.The CPU 502 is in charge of processing received information and sendingrelevant information to the network adapter 503. Additionally the CPUhas the ability to read and write information to embedded storage 504within the mobile device 103. The CPU 502 may use any standard computerarchitecture. Common architectures for microcontroller devices includeARM and x86.

The network adapter 503 is the networking interface that allows themobile device 103 to connect to cellular and WiFi networks. The networkconnection is used in order for the mobile device 103 to access a datasource containing light ID codes 701 with their corresponding locationdata 702. This may be accomplished without a data connection by storinglocation data 702 locally to the mobile device's 103 internal storage504, but the presence of a network adapter 503 allows for greaterflexibility and decreases the resources needed. Furthermore, the networkadapter 503 is also used to deliver location dependent content to themobile device when it is connected to a larger network 601.

FIG. 6 is a representation of multiple LED sources sending light 102 a-dcontaining identification information 102 to multiple mobile devices 103a-103 b. In this instance the light sources are acting as non-networkedbroadcast beacons; there are no networking modules or physical datawires connecting them. This property is desirable when looking towards acommercial installation of numerous LED light sources 103 a-103 b, asadditional wiring and networking will not be required. However, in orderto receive relevant information the mobile devices have the ability tosend and receive additional information from a local source or a network601. Once the mobile device 103 receives identification information 401from the light sources, it then asks a local or remote source foradditional information.

Enclosed area 602 is a spatial representation of an enclosed roomcontaining four LED sources 101 a-101 d and two mobile devices 103 a-103b, meaning that they may operate next to each other withoutinterference. As a rule of thumb if the received image feed from themobile device sees one or more distinct bright sources of light, it hasthe ability to differentiate and receive the unique information withoutinterference. Because the light capture is based on line of sight,interference is mitigated. In this line of sight environment,interference may arise when the light capture mechanism of the mobiledevice is blocked from the line of sight view of the light source.

Network 601 represents a data network that may be accessed by mobiledevices 103 a-103 b via their embedded network adapters 503. The networkmay consist of a wired or wireless local area network (LAN), with amethod to access a larger wide area network (WAN), or a cellular datanetwork (Edge, 3G, 4G, LTS, etc). The network connection provides theability for the mobile devices 103 a-103 b to send and receiveinformation from additional sources, whether locally or remotely.

FIG. 7 describes how the mobile device 103 receives location data 702.In essence, the mobile device 103 sends decoded ID codes 701 through anetwork 601 to a server 703, which sends back location information 702.The decoded ID codes 701 are found in the information 401, which iscontained in the optically transmitted signal. After receiving thissignal containing a unique ID code 701 the mobile device 103 sends arequest for location data 702 to the server 703, which sends back theappropriate responses. Additionally the request could include othersensor data such as but not limited to GPS coordinates andaccelerometer/gyroscope data, for choosing between different types oflocation data 702 and any additional information.

Location data 702 is the indoor location information which matches thereceived information 401. The location data 702 corresponds to indoorcoordinates which match the ID code 701, similar to how outdoor GPS tagsknown locations of interest with corresponding information. The locationdata 702 could also contain generic data associated with the lightidentification information 401. This could include multimedia content,examples of which include recorded audio, videos, and images. Thelocation data 702 may also vary depending, for example, on othercriteria such as temporal criteria, historical criteria, oruser-specified criteria.

The temporal criteria may include the time of day. The historicalcriteria may include user location history (e.g., locations visitedfrequently), Internet browsing history, retail purchases, or any otherrecorded information about a mobile device user. The user-specifiedcriteria may include policies or rules setup by a user to specify thetype of content they wish to receive or actions the mobile device shouldtake based on location information. For example, the user-specifiedcriteria may include how the mobile device behaves when the user isclose to an item that is on sale. The user may specify that a coupon ispresented to the user, or information about the item is presented on themobile device. The information about the item may include videos,pictures, text, audio, and/or a combination of these that describe orrelate to the item. The item may be something that is for sale, adisplay, a museum piece, or any other physical object.

Server 703 handles incoming ID codes 701, and appropriately returnsindoor location data 702 to the mobile devices 103. The handling mayinclude receiving incoming ID codes, searching databases to determinematches, calculating position coordinates based on the ID codes, andcommunicating indoor location data 702. Since the LED light sources 101are acting as “dumb” one-way communication beacons, it is up to otherdevices to determine how to use the ID codes to calculate positioninformation and deliver related content. In some embodiments, the server703 may include the information used to link ID codes 701 to physicalspaces and to deliver location-specific content. The server is designedto handle the incoming requests in a scalable manner, and return resultsto the mobile devices in real-time.

The server may include one or more interfaces to the network that areconfigured to send and receive messages and information in a number ofprotocols such as Internet Protocol (IP) and Transmission ControlProtocol (TCP). The protocols may be arranged in a stack that is used tocommunicate over network 601 to mobile device 103. The server may alsoinclude memory that is configured to store databases and informationused in providing position coordinates and related location basedcontent. The server may include one or more modules that may beimplemented in software or other logic. These modules may performcalculations and perform operations to implement functionality on theserver. The server may use one or more processors to run the modules toperform logical operations.

To describe the server interaction in more detail, FIG. 8 delves intolocation-specific areas 801 containing databases 802 and web services803. The areas 801 represent a subset of databases 802 and web services803 for individual locations where there are installed LED light sources101. The server 703 directly communicates with these installations,which have their own separate sets of information. At a high level,databases 802 represent the stored information pertaining to a specificarea 801, while the web services 803 represent services which allowusers, customers, administrators, and developers access to the ID codes,indoor locations, and other information.

In order to send relevant information, after each received ID code 701,the server 703 requests information pertaining to the specific area 801.Contained in each area 801, are databases which contain informationcorresponding to the specific ID code 701. This information can takemultiple formats, and has the ability to be content specific to avariety of static and dynamic parameters.

In order to optimize response time, the server 703 may constrain itssearch space by using existing positioning technologies available to themobile device 103 or from information in the light source ID codedepending on the embodiment. In essence the server looks for the lightIDs 901 within a specific radius of the current approximate position ofthe mobile device 103, and ignores those that are geographicallyirrelevant. This practice is known as “geo-fencing”, and dramaticallyreduces the request/response time of the server 703. As finalverification, if the database 802 contains one or more of the same IDswithin the current search space that match the ID codes received by themobile device 103 within a specific time frame, then a successfultransaction can be assumed.

As seen in FIG. 9, each database 802 contains numerous sub-categorieswhich store specific types of information. The categories are labeledlight IDs 901, maps 902, content 903, and analytics 904.

Light IDs 901 is a category which contains records of the individuallight ID codes 701 which are contained in an area 801. In a typicallight positioning enabled installation, there will be tens to hundredsof unique LED light sources 101 broadcasting unique ID codes 701. Thepurpose of the light IDs 901 database is to maintain and keep a recordof where the ID codes 701 are physically located in the area 801. Theserecords may come in the form of but are not limited to GPS (latitude,longitude, and altitude) coordinates that are directly mapped into anindoor space. For instance, most indoor facilities have informationabout the number of installed lights, how far apart they are spaced, andhow high the ceilings are. This information may be matched with buildingfloor plans or satellite imagery to create a digital mapping of whereeach light is positioned.

To expand upon the Light IDs 901 category, additional information maycome in the form of location-specific maps 902. These maps may take onmany physical and digital forms, either directly from the management ofthe location, or a third-party vendor or outside source. In addition tomapping information, location-specific content 903 and analytics 904 arealso contained inside the databases 802.

FIG. 10 is a description of the ID log 1001 information contained in theLight IDs database 901. It is a representation of the file structurethat contains individual records corresponding to individual light IDcodes 701 found within different areas 801. In a typical area 801 thereis a possibility of having duplicate ID codes 701 since there are afinite number of available codes. The size of the ID code 701 isproportional to the length of the data 403 field contained in theoptical information 401.

To deal with duplicate ID codes 701, additional distinguishinginformation may be contained inside of the individual log records; ID 11001, ID 2 1003, and ID 3 1004. This information may contain additionalrecords about neighboring ID codes 701 that are in physical proximity ofthe LED light source 101, or additional sensor data including but notlimited to: accelerometer or gyroscope data, WiFi triangulation orfingerprinting data, GSM signature data, infrared or Bluetooth data, andultrasonic audio data. Each additional sensor is an input into aBayesian model that maintains an estimation of the current smartphoneposition and the uncertainty associated with the current estimation.Bayesian inference is a statistical method used to calculate degrees ofprobability due to changes in sensory input. In general, greater numbersof sensory inputs correlate with lower uncertainty.

In order to calibrate the light-based positioning system, a userequipped with a specific mobile application (app) will need to walkaround the specific area 801. The mobile application contains map 902information of the indoor space, with the positions of the LED lightsources 101 overlaid on the map. As the user walks around, they willreceive ID codes 701 from the lights. When the user receives an ID code701, they will use the map on the mobile app to select which LED lightsource 101 they are under. After the user confirms the selection of thelight, the mobile application sends a request to the server 703 toupdate the light location contained in the lighting plan 1103 with theID code 701. Additional user-provided 1104 metadata including but notlimited to current WiFi access points, RSSI, and cellular towerinformation may also be included with the server request to updateadditional databases.

In addition to manual calibration, calibration of LED light source 101locations may also be achieved via crowd-sourcing. In this algorithm, asmobile application users move around an indoor space receiving ID codes701, they will send requests to the server 703 containing the light IDcode 701 received, the current approximate position (based on otherpositioning techniques such as WiFi, GPS, GSM, and inertial sensors) andthe error of the current approximation. Given enough users, machinelearning algorithms on the server 703 may be used to infer the relativeposition of each LED light source 101. The accuracy of this calibrationmethod depends heavily on the number of mobile application users.

FIG. 11 is a description of the maps database 902 and map log 1101information containing floor plans 1102, lighting plans 1103,user-provided information 1104, and aggregated data 1105. Map log 1101is a representation of the file structure that contains the informationfound inside the maps database 902. Information may come in the form ofbut is not limited to computer-aided drafting files, user-providedcomputerized or hand drawn images, or portable document formats. Theinformation residing in the maps database 902 may be used both tocalibrate systems of multiple LED light sources 101, and to augment thelocation data 702 that is sent to mobile devices 103.

Floor plan 1102 contains information about the floor plan for specificareas 801. The contained information may be in the form ofcomputer-aided drafting files, scanned images, and legacy documentspertaining to old floor plans. The information is used to build a modelcorresponding to the most recent building structure and layout. Thesemodels are subject to changes and updates through methods including butnot limited to crowd sourcing models where users update inaccuracies,third-party mapping software updates, and additional input from privatevendors.

Lighting plan 1103 contains information about the physical lightingfixture layout, electrical wiring, and any additional informationregarding the lighting systems in the area 801. This information mayalso come in a variety of physical and digital forms such as the floorplan 1102 information. The lighting plan 1103 information is used in thecalibration process of assigning light ID codes 701 to physicalcoordinates within an area 801. In essence, a location with multiple LEDlight sources 101 acts as a large mesh network except, in this case,each node (light ID 701) is a non-networked beacon of information thatdoes not know about its surrounding neighbors. To help make sense ofmultiple light ID codes 701, the lighting plan 1103 information is usedas one of many ways to tell the backend server 703 where LED lightsources 101 are located.

User-provided information 1104 contains additional data that the usermanually uploads in regards to building changes, updates, or newinformation that is acquired. The user in this case is most likely thefacility manager or staff member, but such information may alsooriginate from an end user of the system who contributes via a crowdsourcing or machine learning mechanism. For instance, if an end user wasusing a light-based positioning system in a museum and was unable tofind a particular exhibit or noticed inaccurate information in regardsto location or classification of the exhibit, they could red flag theoccurrence using their mobile device 103. When coupled with data fromadditional users, sometimes known as a crowd-sourcing method, thisuser-provided information 1104 may be used to update and repairinaccuracies in the maps 902 database.

Aggregated data 1105 contains information that is gathered by the systemthat may be used to augment the current information that is known aboutthe mapping environment. This may occur during normal operation of thesystem where multiple mobile devices 103 are constantly sending andreceiving location data 702 from the server 703. Over time theaggregation of this data may be used to better approximate how light IDcodes 701 correspond to the physical locations of the LED light sources101. For instance, if multiple mobile devices 103 consistently receive anew ID code 701, in a repeatable pattern with respect to additionalknown ID codes 701 and other sources of location information, then thisinformation may be recorded and stored in the aggregated data 1105database. This information may additionally be used to recalibrate andin essence “self-heal” a light-based positioning system.

FIG. 12 is a description of the content database 903 and content log1201 information containing static content 1202, user-based content1203, and dynamic content 1204. Content log 1201 is a representation ofthe file structure that contains the information found inside thecontent database 903. Static content 1202 refers to unchanginginformation that is associated with the specific area 801. This mayrefer to the previous example in which a facility manger loads specificcontent into the content 903 database before a user enters the specificarea 801. This type of information may take the form of but is notlimited to audio recordings, streaming or stored video files, images, orlinks to local or remote websites.

User-based content 1203 refers to content that is dependent on usercriteria. The content may depend on, but is not limited to, user age,sex, preference, habits, etc. For instance, a male user might receivedifferent advertisements and promotions than a female would.Additionally, age and past purchase habits could also be used todistinguish which is the correct piece of content to be presented to theuser.

Dynamic content 1204 refers to content which changes with varyingfrequency. The content may change dependent on a temporal bases, daily,weekly, monthly, etc. For instance, seasonal marketing and content couldbe automatically presented to the user dependent on the month of theyear, or content in the form of morning, evening, or nightly specialscould be presented numerous times throughout the individual day.

In addition to content, point of purchase 1205 information may bedelivered as well. This could be implemented by using the received IDcode 701 to a secure connection that establishes and completes atransaction linked to a user's selected payment method. Additionally, astandalone point of purchase feature could be implemented by simplylinking ID codes 701 directly to merchandise or services.

FIG. 13 is a description of the analytics database 904 and analytics log1301 information containing frequency 1302, dwell time 1303, path taken1304, and miscellaneous 1305. Analytics log 1101 is the file structurethat contains the information found inside the analytics database 904.Frequency 1302 refers to the number of times each end user visits aparticular location inside of a specific area 801. Separate records aremaintained for individual users, and the frequency is aggregated andsorted in the frequency files database 904.

Dwell time 1303 refers to the time spent in each particular locationinside a specific area 801. Separate records are maintained forindividual users, and the dwell times are aggregated and sorted in thedwell time file. Path taken 1304 refers to the physical path taken by auser in each specific area 801.

Consider an example that combines many of the above descriptions,involving a store owner that installed a light-based indoor positioningsystem and a customer walking around the store using a mobile device 103capable of receiving optically transmitted information. The customerdrives to the parking lot of the store, parks, and walks in. Using thebackground sensors and location services available to her phone asmodeled in FIG. 16, the customer's mobile device 103 already knows thatshe has approached, and most likely entered a store outfitted with alight-based positioning system. Once this information is known, theapplication running on the customer's mobile device 103 initiatesseveral background services and begins to start looking for opticalsignals as depicted in FIG. 15.

Prior to the customer entering the store, the store owner has alreadycalibrated and preloaded the database 802 with the unique LED lightsources 101, map 902 information pertaining to the store floor plan1102, user-provided 1104 product locations, and content 903 in the formof multimedia and local deals in the form of promotions that may only beactivated by visiting that particular section of the store.

In the meantime, the customer is walking around the store looking tofind particular items on her shopping list that she has alreadydigitally loaded onto her mobile device 103. Next, the customer isprompted by her mobile device 103 that one of the items on her list haschanged locations and an image of the store layout is displayed with aflashing icon indicating where her desired product has moved. The mobilephone may guide her to the new product. Then as soon as she gets closeto the product, an informational video is prompted on her screendetailing the most popular recipe incorporating that product and how itis prepared. Finally, in addition to finding her desired product, thecustomer receives a discount promotion for taking the time to seek outthe new location of the product.

In addition to the services offered by this system to the customer, thestore owner now gains value from learning about the shopping experiencesof the customer. This comes in the form of aggregated data that iscaptured and stored in the analytics 904 section of his store's database802. This example is one of many applications that may be enabled withan accurate indoor light-based positioning system.

FIG. 14 is a process describing the act of receiving location andcontent information through visible light. User places mobile deviceunder light 1401 corresponds to the act of physically placing a cameraequipped mobile device 103 underneath an enabled LED light source 101.The user stands approximately underneath or adjacent the LED lightsource 101, and the mobile device has the LED light source 101 in viewof the camera lens.

The next block, sample image sensor 1402, refers to the act of turningon and reading data from the embedded image sensor in the mobile device103. Receive ID? 1403 is a decision block which either moves forward ifa location ID is received, or returns to sample the image sensor 1402.Get location data corresponding to ID from server 1404 occurs once alocation ID has been received. The mobile device queries the serverasking for location data 702 relevant to the ID code. This describes theprocess of a user obtaining an ID code 701 from a non-networked LEDlight source 101, and using the unique identifier to look up additionalinformation from either the server 703 or a locally stored source.

Finally, Content? 1405 is another decision block which determines ifthere is location-based content associated with the received ID code. Ifcontent is available the process continues on to the last block 1406where the content is queried; if not, the process ends. As describedabove, the get content data corresponding to ID from server 1405 refersto the act of retrieving content data associated with a known locationfrom either a server 703 or local source.

FIG. 15 is a process describing the act of turning on the applicationbackground services and determining when to sample the image sensor.Initiate background service 1 1501 is the primary background runningservice on the mobile device. This service is tasked with initiating afunction that can communicate wirelessly to determine if the mobiledevice is close to an enabled area. The wireless communication includesradio frequency communication techniques such as global position system(GPS), cellular communication (e.g., LTE, CDMA, UMTS, GSM), or WiFicommunications. Determine position 1502 is the function thatperiodically samples the wireless communication signal and based ondistance parameters decides whether or not the mobile device is closeenough to an area to move forward to the next service.

Light positioning enabled? 1503 is a decision block that moves forwardif the mobile device is close to an enabled location, or repeats theprevious function if not. Initiate background service 2 1504 isactivated once the mobile device enters an enabled area. The service istasked with initiating the functions that receive location informationvia the modulated light.

Sample ambient light sensor 1505 is the first function of the previousservice that samples the ambient light sensor data as soon as the sensordetects a change. The function of this task is to determine if thesensor has gone from dark to light, if the user takes the device out ofa pocket or enclosure, or from light to dark, the user has placed thedevice inside of a pocket or enclosure. As an alternative to samplingthe light sensor, the algorithm could also look for a change in theaccelerometer reading. This may correspond to the user taking the phoneout of their pocket. Detect change? 1506 is the decision block thatmoves forward if the ambient light sensor has gone from dark to light,meaning that the mobile device is potentially in view of surroundingmodulated light.

FIG. 16 is a process describing the act of determining a mobile device'sposition using a variety of information sources. Sample GPS/GSM 1601refers to the act of determining if the mobile device is close to anenabled area. Enabled area? 1602 is a decision block which moves forwardif the mobile device is close to a enabled area, or returns to theprevious block if not.

Sample alternative sources 1603 refers to the act of leveraging existingalternative positioning technologies such as WiFi, Bluetooth,ultrasound, inertial navigation, or employing an existing service usingone or more of any available services. Record internal sensor data 1606is a task which records the current accelerometer data for a period oftime before returning to the Sample image sensor 1402 block. This taskis performed so that location information is constantly being collectedeven when modulated light is not being detected. This allows the mobiledevice and/or server to keep track of the mobile device's position.

FIG. 17 is a system diagram describing how a client device 1704interacts with a light-based positioning system 1709. Network 601 is ageneric local or remote network used to connect mobile devices 103contained in locations A 1701, B 1702, and C 1703 with the light-basedpositioning service 1709.

Each location contains multiple LED light sources 101, each of whichbroadcast unique identification codes 701. In order to interact with thesystem from an operator's perspective, a mobile device may use thedatabase service application 1710 which contains multiple privilegelevels for different levels of access. The client privilege leveldetermines read/write permissions to each of these databases. Theselevels include users 1705 which refer to general front end system users,administrators 1706 which are usually IT or operations management levelwithin an installation, developers 1707 which have access to theapplication programming interfaces of the system for use in customapplication development, and root 1708 level which contains mastercontrol over the users and access to everything contained in the systemand databases.

Mobile devices in each location 1701, 1702, and 1703 receiveidentification codes 701 from lights in their respective locations. Theythen send the received identification codes 701 through the network 601which connects to database service application 1710, through userapplication 1705, and has read access to maps 902 and content, and writeaccess to analytics 904. A generic client, 1704, connects to databaseservice application 1710 through network connection 601.

The client uses a password authorized login screen to access therespective permission status. Clients with administrator permissionshave read/write access to light IDs 901, read access to maps 902,read/write access to content 903, and read access to analytics 904.Clients with developer permissions 1707 have read access to light IDs,read access to maps 902, read/write access to content 903, and readaccess to analytics 904. A client with root permissions 1708 hasread/write access to databases 901-904.

As an overview, FIG. 17 describes the top-down approach to an exemplaryimplementation of a light-based positioning system. At the highestlevel, known locations of installed non-network standalone LED lightsources 101 are used to accurately identify the relative position ofmobile devices 103. In order to obtain identification information fromthe lights, the background processes running on the mobile device 103have been described in FIGS. 14, 15, and 16. Once the mobile device hasacquired a unique or semi-unique ID code 701 from the light orcombination of lights, it uses this information to query a database 802for additional information. This information may come in many forms, andis used to create a more personalized experience for the user. Asinitially mentioned, this local experience is used for location-awaremobile computing, and augmented reality applications. In addition tolocal personalized information, location-based analytics applicationsmay be enabled from the aggregated data and traffic running through theserver 703.

The use of light-based positioning capabilities provide a number ofbenefits. For example, the positioning information obtained by usinglight sources is highly precise compared to alternative techniques forpositioning information. The accuracy of a light-based positioningsystem may be down to a few centimeters in three dimensions in someembodiments. This positioning ability enables a number of usefulservices to be provided. In certain embodiments, additional mobiledevice information may be used in combination with the positioninginformation. For example, accelerometer position information may be usedin conjunction with light source based position to offer augmentedreality or location aware content that relevant to the device'sposition. The relevant content may be displayed to augment what is beingdisplayed on the mobile device or the display can provide relevantinformation. Applications on the mobile device may also be launched whenthe mobile device enters certain areas or based on a combination ofcriteria and position information. The applications may be used toprovide additional information to the user of the mobile device.

The light-based positioning systems and techniques may also be used tomanage and run a business. For example, the light-based positioning mayhelp keep track of inventory and to make changes to related databases ofinformation. In a warehouse, for example, the light-positioning systemmay direct a person to where a particular item is located by givingdirections and visual aids. The light positioning may even providepositioning information to direct the person to the correct shelf theitem is currently residing on. If the person removes the item, themobile device may update the inventory databases to reflect the change.The same function may be implemented in a store environment asmerchandise locations are changed or updated. This information may thenbe used in providing content to a user. For example, if a shopper wantsmore information about an item, the updated location may be used tolocate the item or direct the shopper to an online website to purchasean out-of-stock item. In some embodiments, the mobile device using thelight-based positioning technique in conjunction with a wirelessconnection and other information may be used to provide non-intrusivedata collection on customers. The data collection of how customers movethrough a store and where they spend time may be used to improve layoutof stores and displays of merchandise.

The light-based positioning systems are also easy and low-cost to set upcompared to other location-positioning systems. Since each light sourceoperates autonomously, a building owner only needs to swap out existinglight sources for those that provide light-based information to acamera-enabled device. The light sources are non-networked independentbeacons that broadcast identification codes configured whenmanufactured. This allows the light sources to be manufactured at alower cost compared to networked light sources. Further, thenon-networked independent beacon light sources in the light-basedpositioning system may be easier for building owners to install.

The light-based positioning system may also include optimizations insome embodiments. For example, location information obtained from eitherthe identification code or from alternative techniques can be used toreduce latency in determining position information. This optimizationmay work through geo-fencing by constraining the search area to findinformation regarding the captured light sources more quickly. This canreduce the overall delay experienced by a user from the time the mobiledevice captures the light sources to when relevant position informationis provide to the mobile device and/or relevant content is provided tothe mobile device.

Efficient Light Bulbs for DPR Schemes

One of the biggest challenges facing beacon-based light-positioningsystems is managing the additional power consumption ofcommunication-enabled lighting devices in comparison to that ofnon-communicating devices. Lighting sources 101 in general, regardlessof form factor or technology, are differentiated in part by their powerconsumption; generally, the less the better. Accordingly, higher energyefficiency is one of the core economic forces driving adoption ofLight-Emitting-Diodes (LEDs). However, when using light sources 101 as ameans for communication devices, the power requirements tend to increasedepending on the modulation scheme since energy must be divided betweenthe carrier wave and the modulation wave. There are many differenttechniques for transmitting data through light, for example, asdiscussed in U.S. Ser. Nos. 12/412,515 and 11/998,286, and 11/591,677,the entire disclosure of each of which is incorporated by referenceherein. However, these techniques have primarily been pursued withoutconsidering their impact on light source 101 parameters, includingefficacy, lifetime, and brightness. Since light sources 101 are firstand foremost illumination devices, and not communication devices, thecommunication function takes a secondary role. The present disclosureutilizes Digital Pulse Recognition (DPR) modulation as a technique fortransmitting data while minimizing the impact on illumination devices.

FIGS. 18A-C represent several digitally modulated light sources 101 a-cwith varying duty cycles; a low duty cycle 1801, a medium duty cycle1802, and a high duty cycle 1803. A duty cycle is a property of adigital signal that represents the proportion of time the signal spendsin an active, or “on,” state as opposed to an inactive, or “off,” state.A light source with a low duty cycle 1801 is inactive for a highproportion of time. A light source with a medium duty cycle 1802 isinactive for about the same proportion of time that it is active. Alight source with a high duty cycle 1803 is active for a high proportionof time. The duty cycle of a light source affects the luminosity of thelight source. A light source having a higher duty cycle generallyprovides more luminosity than that same light source with a lower dutycycle because it is on for a higher proportion of time. Duty cycle isone aspect of a modulation scheme. Other aspects include pulse shape,frequency of pulses, and an offset level (e.g., a DC bias).

Because DPR modulated light sources 101 rely on frequency modulation,they are able to circumvent the limitations of traditional AM basedapproaches. Note that frequency modulation in this context does notrefer to modifying the frequency of the carrier (which is the lightsignal), but instead to modifying the frequency of a periodic waveformdriving the light source. One popular technique for dimming LED lightsources 101 is pulse-width modulation (PWM), which controls the averagepower delivered to the light source by varying the duty cycle of apulse. In a DPR modulation system utilizing PWM, a DPR modulator wouldcontrol the frequency of the pulses, with the duty cycle determined bythe dimming requirements on the light source 101. As used herein, aDPR-modulated light source, having a DPR modulation frequency, refers toa light source having an output modulated in such a manner that areceiver using DPR demodulation techniques may demodulate the signal toextract data from the signal. In some embodiments, the data may includeinformation in the form of an identifier that distinguishes a lightsource from other nearby DPR-modulated light sources. In someembodiments, this identifier is a periodic tone that the light sourcerandomly selects to identify itself. A periodic tone may be a signalthat repeats with a given frequency. In other embodiments, a lightsource may receive such an identifier from an external source.

To determine the maximum duty cycle (D) supported by DPR demodulation,the modulation frequency (f) of the transmitter and the sampling timefor the image sensor (T_(s)) of the receiver are first defined. Next theduty cycle parameters (T_(off)) and (T_(on)) that correspond to the onand off times of the light source are defined. T_(s) is an importantparameter because the image sensor sampling time defines a minimumamount of modulation time required to produce the banding effects whichallow for the frequency detection required for DPR demodulation. Therequired modulation time may refer to either the T_(on) portion 1804 orthe T_(off) portion 1805 of the signal; however, to maximize thebrightness of the light source, T_(off) is used as the limiting variable(if solving for the minimum duty cycle, T_(on) may be used). If T_(s) ofthe receiving device is less than twice T_(off) of the light source,residual banding on the image sensor will typically not take place;therefore, the signal cannot be extracted. In order for banding tooccur, T_(s) should be greater than twice the value of T_(off)(T_(s)>2×T_(off)).

It is important to note that when designing for the maximum duty cycle,the modulation frequency may be defined from the transmitter side andmay be completely independent of the sampling time T_(s). This isbecause the sampling frequency T_(s) is a property of the receiver,which is defined by the image sensor manufacturer and is likely notdesigned for optimal DPR demodulation properties. T_(s) varies dependingon the specific image sensor, and may be expected to change as moreadvanced image sensors are developed. Therefore, it is important tooptimize such that a broad range of both modulation and samplingfrequencies may be used. In the next sections the equations andvariables for the calculation of the maximum duty cycle are describedfor a variety of test cases.

In order to solve for T_(off) in terms of duty cycle and modulationfrequency, one may first start with the fundamental definition of whatthe duty cycle is: 1 minus the ratio of signal on time divided by thecombination of signal on and off time. In the case of a modulated lightsource, D=1−T_(off)/(T_(on)+T_(off)). Next, the modulation frequency (f)may be defined as the inverse of the sum of signal on and off times:f=1/(T_(on)+T_(off)). Substituting f into the previous equation for Dyields D=1−f×T_(off). The variable T_(off), which was previously definedas a value less than twice T_(s), may then be used to define the maximumduty cycle for any given modulation used in DPR demodulation. Afterrearranging and substituting T_(s) for T_(off) (T_(off)<0.5×T_(s)),D=1−f×(½)×(T_(s)). With this equation, one may now solve for the maximumduty cycle achievable given the modulation frequency of the transmitter,and the sampling time of the receiver.

Since the maximum duty cycle is dependent on both the modulationfrequency of the transmitter and the sampling frequency (F_(s)=1/T_(s))of the receiver, its exact percentage value may change depending on thepresent conditions. For testing purposes, the modulation frequency rangewas chosen to start at 300 Hz, which is above the range which the humaneye can see. The modulation frequency range may range from 60 Hz to 5000Hz. Typical image sensor sampling frequencies (F_(s)=1/T_(s)) rangebetween 20 kHz and 36 kHz for high-quality image settings (640 by 480pixel resolution), and 4 kHz to 7 kHz for low-quality image settings(192 by 144 pixel resolution). In some embodiments, the image sensorsampling frequencies may range from as low as 1 KHz to as high as 1 MHz.

When analyzing specific use cases, the duty cycles corresponding to amodulation frequency of 300 Hz and sampling frequencies for high-qualityimage settings in some embodiments result in D=1−(300 Hz)×(½)×( 1/20Khz)=99.25% and D=1−(300 Hz)×(½)( 1/36 kHz)=99.58%. The duty cyclescorresponding to a modulation frequency of 300 Hz and typical samplingfrequencies low-quality sampling frequencies in other embodiments resultin D=1−(300 Hz)×(½)×(¼ kHz)=96.25% and D=1−(300 Hz)×(½)×( 1/7kHz)=97.86%. In yet other embodiments, a 2000 Hz modulation frequencyand high-quality sampling frequencies of 20 kHz and 36 kHz results inD=95.00% and 97.22% respectively, and for low-quality samplingfrequencies of 4 kHz and 7 kHz results in D=75% and 85.71% respectively.

After the maximum duty cycle has been calculated, to compensate for theadditional power requirements needed for data communication due to theoff portion 1804 of the modulation signal, the input power may beincreased such that the resulting average power of the communicatinglight source 101 is identical to the non-communicating light source 101.In effect, the average power of the two light sources will be the same,yielding a perceivably identical luminous output. Take for instance LEDsource “A” that is powered by 6 watts and modulated where 50% of thetime it is “on”, and the remaining 50% “off”, effectively resulting in a3-watt average power. In order for this light source 101 to match theluminous output of the 6-watt LED source “B” that is not modulating andis on 100% of the time, one may double the input power from 6 watts to12 watts. While the input power of “A” was increased, the average poweris halved to equal 6 watts; therefore, sources “A” and “B” appear to beidentical to the human eye in terms of brightness.

However, there exists a point where increasing the input power maydecrease the efficiency of a given light source 101. For LED lightingdevices it is important to stay within the manufacturer-specifiedvoltage and, more importantly, current, otherwise efficiency drasticallyfalls with increased supply current. This unwanted effect is known asLED “droop,” and generally refers to decreased luminous output for anygiven individual LED (assuming one or more LEDs per lighting source 101)due to the additional thermal heating resulting from the increasedcurrent. In the previous example, the input power to LED source “A” wasdoubled while the input power to “B” was left unchanged. Assuming thateach source was supplied by a constant 12 volts, this means that theinput current to source “A” had to have doubled in order to achieve therequired 12 watts of power consumption. This equates to a 50% increasein current, when moving from 0.5 amperes to 1 ampere, and may only beperformed if within the manufacturers' tolerable input current range forthe LEDs.

Given inputs of drive current (Id) and operating voltage (V), one maydefine the power (P) of a non-modulated light source 101 as P=Id×V, andcompare it with the additional required power (P_(mod)) of a modulatedlight source 101. To define the additional power needed due tomodulation, one may then define the relationship as P_(mod)=P2−(D×Id×V).While the input variables used in this example vary from source tosource, this method may be used to accommodate for power loss due tomodulation.

One may now solve for the power required to support the maximum dutycycles that were previously solved for. In this example, the powerconsumed by the non-modulated light source equals P=Id×V=700 mA×12 V=8.4W. P_(mod) may then be calculated to describe how much extra power isrequired to support a modulated light source 101 with regard to the dutycycle. Recall that for a modulation frequency of 2000 Hz and samplingfrequencies of 20 kHz and 4 kHz, the maximum duty cycle equaled 99.25%and 96.25%. Therefore, the additional power needed to detect a 2000 Hzsignal at a sampling frequency of 20 kHz is defined as Pmod=8.4W−(0.9925×70 mA×12 V)=63 mW, a 0.75% increase in required power on topof the baseline 8.4 W. For 2000 Hz at a sampling rate of 4 kHz,P_(mod)=8.4 W−(0.9625×700 mA×12 V)=315 mW, a 3.75% increase in requiredpower.

While finding the maximum duty cycle supported by DPR demodulation isimportant for maintaining the brightest luminous output levels, it isalso important to support the lowest duty cycle possible in order tosupport the dimmest luminous output levels. This is because the minimumduty cycle corresponds to the dimmest level at which a modulated lightsource 101 may operate at while still supporting DPR demodulation from areceiving device. In order to account for this, one may consider theT_(on) portion of the signal rather than T_(off). The limiting samplingfactor now changes to require that T_(s) is greater than twice T_(on)(T_(s)>2T_(on)). Substituting this condition into the previous max dutycycle equation (replacing {1−D} with D), the resulting equation yieldsD=(½)×f×T_(s).

Repeating the above examples for a modulation frequency of 300 Hz andhigh-quality sampling frequencies (1/T_(s)) of 20 kHz and 36 kHz,D=0.75% and 0.42%, respectively. For a modulation frequency of 2000 Hzwith high-quality sampling frequencies, D=5.00% and 2.78%. Consideringlower-quality sampling frequencies at 300 Hz and 2000 Hz, D=3.75% and2.14% for a 300 Hz modulation frequency, and D=25.00% and 14.29% for a2000 Hz modulation frequency.

In addition to modifying the overall duty cycle, there also exists theopportunity to tune the modulation scheme such that during the “off”portion 1805 of operation the light source 101 does not turn completelyoff. As described in FIGS. 19A-C, modulation schemes 1901, 1902, and1903 depict varying duty cycles where a DC bias 1904 has been addedwhich correspond to the modulated light sources 101 a-101 c. Modulationschemes where the light source 101 does not turn all the way “off” areimportant when considering light source 101 brightness, efficiency,lifetime, and the signal to noise ratio (SNR) of the communicationschannel. The DC bias 1904 during modulation reduces the peak powerrequired to drive the light source for a given brightness. A reductionin peak power will reduce the negative impact of overdriving thelighting source, which is known to cause efficiency losses known as“droop” for LEDs, in addition to decreasing light source 101 lifetimes.

As an example, consider that the average power delivered to the lightsource is defined as: P_(av)=D×P_(on)+(1−D)×P_(off), where D is the dutycycle and P_(on), P_(off) are the respective on/off powers. The impacton light source 101 brightness is that increasing the “off” power willincrease the total power. This reduces the required peak power deliveredto the lighting source, because the power transferred during the “off”period can make up the difference. In a system operating at a duty cycleof 50%, for a fixed brightness B, a 10% increase in the “off” periodpower translates to a 10% decrease in the “on” period power.

When approaching the above power equation from a constant voltage (V),average current (I_(av)), and on/off current (I_(on)/I_(off)) standpoint(P=IV), I_(av)×V=D×I_(on)×V+(1−D)×I_(off)×V. After removing the constantV, I_(av)=D×I_(on)+(1−D)×I_(off). For example, in the case of a lightsource 101 requiring an average drive current (I_(ave)) of 700 mA andoff current of (I_(off)) of OA undergoing modulation with a duty cycle(D) of 96.25%, the peak current (I_(on)) requirement is I_(on)=700mA/0.9625=727 mA. If instead the current delivered during the “off” timeis 100 mA the average current reduces to I_(av)=0.9625×700mA+(1−0.9625)×100 mA=678 mA, a 6.7% decrease in overall required powergiven constant voltage. In other embodiments, a constant current may beapplied with differing voltages to achieve a similar effect.

The impact of non-zero I_(off) values for the previous example istwo-fold. First, a reduction in required power is achieved, and secondincreasing the “off” time power lowers the required duty cycle toachieve a fixed brightness level. For the previous example when solvingfor D, D=(I_(av)−I_(off))/(I_(on)−I_(off)). The difference in duty cyclemay now be determined for the reduction in peak current from 727 mA to678 mA, as D=(700 mA−100 mA)/(727 mA−100 mA)=95.69%, which is a 0.56%difference from 96.25%. This essentially allows for a brighter lightsource 101 with a decreased duty cycle, and lower power requirements.

Another major requirement for DPR modulation is to interface withexisting light dimmers. There are a variety of light source 101 dimmersemployed on the commercial market. One popular dimming technique istriac dimming. In a triac dimmer, a variable resistor switch is used tocontrol the amount of power delivered to the light source 101 over theAC line. For traditional incandescent and fluorescent sources this is acost-effective and efficient way to control the power, and thus thebrightness, delivered to the light source 101. For LED light sources101, it is necessary to put a special driver between the triac dimmingcircuit and the LED source. This is because LEDs are current-drivendevices, and thus require an AC/DC converter to transform AC from thepower lines to a DC current for driving the LEDs.

FIG. 20 demonstrates a system by which a DPR modulator may interfacewith existing lighting control circuits. A dimmer controller 2002 sendsa dimmer signal 2003 to a dimmable LED driver 2006. In the case of anLED light source controlled by a triac dimmer, the dimmer signal wouldbe transmitted across the AC power line. The dimmable LED driver 2006then converts the dimmer signal to a pulse width modulated signal usedfor driving the light output 2007 of the source 2001. The configurationof the system diagram shows the dimmer signal 2003 going to both the DPRmodulator 2004 and the LED driver 2006; however, this does not alwaysneed to happen. In some instances the LED driver 2006 may contain a“master override” input that is designed to supersede any dimmer signal2003 input. In this case, the dimmer signal 2003 still goes to the LEDdriver 2006, but is ignored. In other cases where there is not anoverride input, the dimming signal only goes to the DPR modulator.

DPR modulator 2004 is responsible for sending DPR signals 2005 to theLED driver 2006 that controls the light output 2007. In the case of thelight source 2001 being driven by pulse-width modulation as the dimmersignal 2003 from the dimmer controller 2002, DPR modulator 2004 controlsthe frequency of the PWM signal and selects the desired value. The widthof pulses in signals 1801-1803 are determined based on dimmer signal2003, which indicates the desired light source 2001 brightness level.Note that the dimmer controller 2002 is not contained within the lightsource 2001, and may output a variety of dimmer signals 2003 (triac, ora proprietary method). Because of this, the DPR modulator 2004 isresponsible for interpreting these different signals and appropriatelyoutputting a DPR signal 2005 that corresponds to the desired brightnesslevel of the inputted dimmer signal 2003. In cases where dimming is notrequired and the dimmer signal 2003 is not present, the DPR modulator2004 interfaces directly with the LED driver. In some implementations,the DPR modulator 2004 may also be contained inside the LED driver 2006as part of an integrated solution instead of as a separate component.

FIG. 21 contains a high level overview of a DPR modulator 2004. Data2101 is first sent to DPR tone generator 2102. Data 2101 may containinformation from any source. In the context of a beacon-basedlight-positioning system, data may include the identifier for the light.DPR tone generator 2102 converts the data 2101 into a sequence of DPRtones. A DPR tone is a periodic digital signal that oscillates betweenactive and inactive states with a particular frequency. This process isdescribed further in FIG. 22. Depending on the requirements of the datatransmission channel, this could either be a single tone (suitable for abeacon based positioning system using light identifiers), or a sequenceof tones (if higher data rates are desired by the end user). The DPRTone(s) 2203 are then sent to the waveform generator 2103, which isresponsible for generating the DPR signal 2005 for driving the LEDs.Waveform generator 2103 receives a dimmer signal 2003 input from adimmer controller 2002, which controls the brightness of the lightsource. In the case of a DPR tone as a pulse-width-modulated signal,dimmer controller 2002 would control the duty cycle of square wave 1802,while DPR Tone(s) 2203 would control the frequency of the square wave.The result is an output DPR signal 2005, which is then sent to the LEDdriver 2006.

FIG. 22 contains a breakdown of DPR Tone Generator 2102. This module isresponsible for taking a piece of data and converting it to a sequenceof DPR tones. A DPR tone determines the frequency at which a waveform,such as the square waves from FIG. 18, is sent. The range of possibletones, defined here in as T_(o) through T_(n), is determined by both thesampling time, T_(s), of the image sensor (as discussed), and thefrequency response of the light source 101. Encoder 2201 is a standardbase converter—it takes a piece of data in binary and converts it into acorresponding DPR tone. A typical range for tones created by DPR ToneGenerator 2102 is 300 Hz-2000 Hz, in steps of 10 Hz, allowing for 170distinct DPR tones. The step size between tones is selected to reducenoise, and depending on the requirements could be much higher or lowerthan 10 Hz. As an example, that data 2101 may contain an identifier ofvalue 10 for light source 101. This identifier is passed to Tone(s)Generator 2102, which generates (or selects from memory) a sequence oftones. Note that the length of a DPR tone sequence could be as low as 1(in the case of a single tone used in a beacon-based positioningsystem). In this example, an identifier of 10 would map to a DPR tone of400 Hz. DPR Tone Generator 2102 could either store the identifier inmemory beforehand, using pre-computed mappings of data to tonesequences, or alternatively it could compute this on the fly. The exactmethod of generating the sequence of tones may be driven by theresources available on the light source 101. Once one of the possibletones sequences 2202 is created, it is sent to Waveform Generator 2103.

FIG. 23 contains the breakdown of Waveform Generator system 2103, whichcombines a tone sequence 2202 with a waveform from symbol creator 2303and dimmer signal 2003 to create a DPR signal 2005 for driving lightsource 101. The resulting waveform will be periodic, with a frequencydefined by the sequence of tones, a symbol created based on the list ofpossible symbols in symbol creator 2303, and an average output(brightness) determined by the dimmer signal 2003. This desiredbrightness could either be hard-coded on the module, or provided as anexternal input through a dimming control module. The choice of a symbolis determined within Symbol Selector 2301, which generates a controlline 2302 for selecting a symbol from symbol mux 2402.

FIG. 24 contains the breakdown of Symbol Creator 2303, which holdspossible symbols 2401 a-2401 d. These could include a saw tooth wave2401 a, sine wave 2401 b, square wave 2401 c, and square wave with a DCoffset 2401 d, or any other periodic symbol. Symbol creator then takesin a selected symbol 2402, and modifies it such that a desiredbrightness 2106 is achieved. In the case of a square wave symbol 2401 c,dimmer signal 2003 would modify the duty cycle of the square wave. Theresulting waveform is then sent to output signal 2005 for driving thelight source.

The goal of the output waveform 2105, which drives light source 101, isto illuminate a scene in such a way that the DPR modulated signal may bepicked up on any standard mobile device 103. Reducing flicker on videowhich is under illumination from fluorescent lamps is a well-knownproblem. The flicker is caused by periodic voltage fluctuations on theAC line powering the lamp. For a lamp powered by a 50 Hz AC line, theluminance level changes at 100 Hz. This causes alternating white/darkbands to appear in video recorded with CMOS imagers. The bands are aresult of the rolling shutter mechanism on CMOS imagers, which partiallyexpose different areas of the image at different points in time. Thelines on the image may occur on both, one, or on multiple frames, andmay appear to move in time. See, for example, U.S. Pat. No. 6,710,818,the entire contents of which is hereby incorporated in its entirety,which describes methods for detecting and removing this unwanted effect.Possible algorithms for mitigating flicker include automatic exposurecontrol, automatic gain control, and anti-banding. These techniques arecommon in many mobile devices as a means to remove flicker caused byfluorescent lamps.

Advanced DPR Demodulation Techniques

DPR demodulation, instead of removing flicker, exploits the rollingshutter effects of CMOS cameras as a means of transmitting data. A CMOSdevice with a rolling shutter captures an image frame by sequentiallycapturing portions of the frame on a rolling, or time-separated, basis.These portions may be vertical or horizontal lines or “stripes” of theimage that are captured at successive time intervals. Because not everystripe is captured in the same time interval, the light sourcesilluminating the image may be in different states at each of these timeintervals. Accordingly, a light source may produce stripes in a capturedframe if it is illuminated in some time intervals and not illuminated inother time intervals. Light sources that broadcast digital pulserecognition signals may produce patterns of stripes. Since the patternof stripes is dependent on the frequency of the digital pulserecognition signal, and the speed of the rolling shutter can bedetermined a-priori, image processing techniques may be used to deducethe illumination frequency based on the width of the stripes. Forexample, consider a room containing five light sources 101, eachbroadcasting at 500 Hz, 600 Hz, 700 Hz, 800 Hz, and 900 Hz,respectively. Each distinct frequency, otherwise known as a DPR tone,may be used to identify the light source 101. In a beacon-basedlight-positioning system, a mobile device receiver within view of thetransmitting lights can detect the DPR tones, correlate an identifierassociated with the tone, and then use a lookup table to determine thelocation of the device based on the location associated with theidentifier(s).

Modeling the camera sampling function is advantageous in understandinghow DPR demodulation works on modern image sensors, and how the impactsof various hardware-dependent parameters affect the DPR signal 2105. Torepresent this, FIG. 25 is a continuous time representation 2501 of howan individual row on a rolling shutter image sensor is sampled. Theexposure time interval 2502 represents the period over which lightaccumulates on the photo sensor. If the exposure time is much lower thanthe period of the DPR modulated signal, the light and dark bands will beclearly defined. If the exposure time is longer, the light and darkbands will lose their definition.

FIG. 26 contains a continuous time example 2601 of a DPR modulated lightsignal. In this example, the signal is a square wave with a 50% dutycycle being driven at a DPR tone of 300 Hz. The relationship between theDPR illumination period 2602 and the exposure time 2502 determines howwell defined the bands are on the received image.

FIG. 27 is the continuous time sampled image 2701, created by convolvingan individual row sampling function 2501 with a DPR modulated signal2601. The alternating periods of high brightness 2702 and low brightness2803 are caused by the DPR modulation frequency, and appear asalternating white/dark bands on the received image.

FIG. 28 is a representation of a discrete time-domain signal model 2801for representing how a rolling shutter on an image sensor samples theincoming light pulses 2601. The rolling shutter is modeled as an impulsetrain, containing a sequence of the Dirac Delta functions (otherwiseknown as a Dirac comb). Each impulse is separated by an interval, T,which corresponds to the speed of the rolling shutter commonly found inmost CMOS image sensors. The interval T varies from device to devicewhich causes the bands on scenes illuminated by DPR modulated signals tovary in size. The mobile device 103 preferably accounts forhardware-dependent factors (e.g., rolling shutter speed) to properlydetermine the DPR tone. FIG. 29 contains a discrete time representation2901 of the rolling shutter sampling functionality over multiple frames.

Because rolling shutter speeds are typically faster than frame rates,DPR demodulation on current imaging technology is capable of much higherdata rates than modulation schemes that sample on a per-frame basis. Ina DPR modulated system using a 640×480 pixel image sensor, the sensorwould capture 480 samples per frame (represented as 480 consecutivedelta functions in sensor model 2801). A demodulation scheme using aglobal shutter would only be capable of taking one sample per frame.This is a key advantage for indoor positioning using beacon-basedbroadcasting schemes because the time-to-first-fix is orders ofmagnitude faster than competing technology, which may take severalseconds to receive a signal. For example, consider a typical mobiledevice 103 camera which samples at 30 frames per second (FPS). Using DPRdemodulation, time-to-first-fix may be achieved with as little as asingle frame, or 1/30 of a second, versus 1 second for a demodulationscheme that samples on a per-frame basis. This compares to atime-to-first-fix of up to 65 seconds for GPS, 30 seconds for assistedGPS, and 5-10 seconds for WiFi positioning.

This order of magnitude improvement opens the door for applications inwhich latency for time-to-first-fix must be minimized. Furthermore,computation for DPR demodulation may be performed on the mobile deviceitself, versus the server-side processing required for WiFifingerprinting algorithms. In a mobile environment, where connection toa network is not guaranteed, client-side processing provides a majoradvantage. In the future, it is expected that image sensors will havemuch higher frame rates. In this scenario, DPR demodulation may beadjusted to sample on a per-frame basis, instead of a rolling shutterbasis. The key principle is that the demodulator may be adjusted insoftware, allowing future mobile devices to tune their receivingcharacteristics to receive DPR signals. The software adjustments thatneed to be applied are the subject of the following sections.

Configuring a Device for DPR Demodulation

In order to prepare a mobile device 103 to receive the modulated DPRsignals 2105, the device is first configured. This is to counteract theflicker-mitigation algorithms typically applied in mobile device imagesensors. FIG. 30 describes the method by which mobile device 103 isconfigured to receive DPR modulated signals. First, the initializesensors 3001 function initializes and activates the available sensorscapable of receiving data. For typical modern mobile devices these wouldinclude both the front- and rear-facing cameras. Here, a “front-facing”camera or other sensor of a mobile device is one that is mounted on thesame side of the device as its display and is therefore likely to facetoward a user. In one preferred embodiment, the rear-facing camera oranother rear-facing is used because it is more likely to have a view ofthe user's surroundings that is relatively unoccluded by the user's ownbody and thus to record light cast directly by local light sources.Determine sensors to modify 3002 then decides which sensors need to bemodified. A number of possible factors determine whether or not aparticular sensor should be initialized then modified, including powerconsumption, accuracy, time since last reading, environmentalconditions, required location accuracy, and battery state.

Modify sensors 3003 then passes a list of the appropriate sensors whichneed to be modified to a function which has additional information aboutthe mobile device 103 and adjusts the demodulation scheme for devicespecific limitations 3004. In the case of using an embedded mobiledevice 103 camera to demodulate DPR signals, possible sensor parametersto modify include exposure, focus, saturation, white balance, zoom,contrast, brightness, gain, sharpness, ISO, resolution, image quality,scene selection, and metering mode. As part of the modification step3003, sensor parameters such as exposure, white-balance, and focus arelocked to prevent further adjustments.

After the sensors are modified 3003, specific hardware limitations areadjusted for in the demodulation scheme by using a device profile. Themost important of these is the rolling shutter speed. Because differentmodels of mobile device 103 will, in general, have different camerasensors, the line width of the DPR tone measure on an image sensor willvary across hardware platforms for a fixed frequency. For this reason,it is necessary to adjust the stripe width one is looking for dependingon the specific characteristics of the device. In the Fourier Techniquesdiscussed later on in the application, modifying the stripe widthcorresponds to modifying the sampling frequency of Dirac Comb 2801.

There are a number of challenges associated with controlling the cameraparameters to optimize for DPR demodulation. One challenge is overridingthe automatic parameter adjustments that mobile operating systemstypically provide as part of their camera application programminginterfaces (APIs). In the case of an embedded image sensor, the sensorsettings are adjusted automatically depending on factors such as but notlimited to ambient light conditions, areas of focus, distance fromobjects, and predetermined scene selection modes. For instance, whentaking a picture with an image sensor, if the scene is dark then theexposure time is automatically increased. When taking picture of a scenemode with fast moving objects, the exposure time is usually decreased.

When using an image sensor for DPR demodulation, these automaticadjustments may introduce noise into the signal, causing higher errorrates. Specifically in the case of exposure, longer exposure timescorrespond to lower data rates, which correspond to a decreased amountof available light IDs 901. At the edge case, if the exposure time issufficiently long, then the sampling rate will drop so low that DPRdemodulation becomes extremely challenging as the signal is severelyunder-sampled. Furthermore, if the camera is constantly adjusting, thenthe performance of background subtraction (discussed later), whichisolates the moving stripes from the rest of the picture, will besignificantly impaired. This is because the automatic adjustments areconstantly changing the pixel values. In order to successfully transmitDPR signals, these automatic adjustments need to be accounted for.

Practically speaking, many mobile device 103 APIs do not allow for themodification of sensor parameters in the top-level software. Theproposed method in FIG. 31 describes a method for working around theprovided APIs to control the exposure. Current APIs do not allow formanual exposure control, so instead of manually setting the exposure, analgorithm is presented that exploits the metering functionality tominimize the exposure time.

FIG. 31 contains a process for modifying the various sensor parameterscontained in a mobile device 103 in a way that overcomes the limitationsimposed by current camera APIs. In the algorithm, the first step is toinitialize the required sensors 3001. For the case of an image sensor,this involves setting the frame rate, data format, encoding scheme, andcolor space for the required sensors. After the image sensors have beeninitialized 3001, the algorithm searches for regions of interest 3101.In the case of setting the exposure using metering, these regions ofinterest 3101 would be the brightest regions of the image. Set meteringarea 3102 then sets the metering area to the brightest portion,effectively “tricking” the mobile device 103 into lowering the exposuretime. Lock parameter 3103 then locks this exposure time to prevent theauto-adjustment feature of the camera from overriding the manualsetting. Next, adjust for hardware dependent parameters 3104 accesses alookup table and adjusts the demodulation algorithm based on hardwareand software differences. For the case of an image sensor, one exampleof this is changing the sampling time based on the rolling shutter speedof the device. This rolling shutter speed may either be loaded from alookup table beforehand (using predetermined values) or measured on thefly. Each device only needs to measure its rolling shutter speed onceper image sensor. Once parameters set? 3105 is satisfied the algorithmends; otherwise, it returns to identify regions of interest 3101.

The method of exploiting the metering area on a mobile device 103 may beused to optimize many of the required parameters in addition to theexposure, including white balance, contrast, saturation, ISO, gain,zoom, contrast, brightness, sharpness, resolution, image quality, andscene selection. Furthermore, these parameters could already be knownbeforehand, as each mobile device 103 will have its own “device profile”containing the optimal camera settings. This profile could be loadedclient side on the device, or sent over a server. Note that although themethod of using the metering area to control the exposure may improvethe performance of DPR demodulation, it is not strictly necessary.Simply locking the exposure 3103 is often sufficient to prevent theautomatic camera adjustments from filtering out the DPR signals.

Advanced Techniques for Decoding Information in DPR Modulated Signals

Once the sensors have been initialized 3001 and parameters have been set3104, FIG. 32 describes a process for decoding the information containedinside a DPR modulated signal. Identify regions 3201 is used to separatedifferent regions on the image illuminated by DPR signals. At the baselevel, the region of interest is the entire image. However, when one ormore light sources 101 are present, there exists an opportunity toreceive multiple DPR signals simultaneously. In this scenario, thesensor effectively acts as a multiple antenna receiver. Such multipleantenna systems, more generally referred to as multiple-inputmultiple-output (MIMO), are widely used in the wireless networkingspace. This is an example of spatial multiplexing, where wirelesschannels are allocated in space as opposed to time or frequency. Theimplications of MIMO for DPR demodulation in a beacon-basedlight-positioning system is that frequencies may be re-used in a spacewithout worry of interference. When a mobile phone user receives DPRmodulated signals on a photodiode array (such as an image sensor, or anyimaging technology that contains multiple spatially separated sensors),the DPR signals will each appear at different locations on the sensor.Each region 3201 of the image may then be processed independently, inthe same way that each mobile phone user in a cell network only connectsto the cell they are closest to.

This works in a way analogous to cellular phone networks. With cellularnetworks, mobile phone users only communicate with cellular towers thatare close to them. This allows multiple mobile phone users to share thesame frequency, provided they are all on different cells. In DPRmodulation, each light acts as its own cell transmitting uniquefrequencies. However, different lights may also use the same frequencyprovided that they are far enough apart. Re-using the same frequenciesin different space allows for greater system scalability, since lightingsources 101 may be installed at random without requiring the installerto worry about frequency allocation.

After sensors have been initialized 3001, and regions of interest 3201have been identified, detect frequency content 3202 identifies thepresence of DPR tones from the sensor data. Described here are multiplemethods for extracting the frequency content from a DPR signal. Onepossibility is to use line-detection algorithms to identify the pixelwidth of the stripes, which directly corresponds to the transmittedfrequency. This stripe width is then used to access a lookup table thatassociates width and transmitted frequency and determines thetransmitted tones. Possible methods for detecting lines include Cannyedge detection, Hough Transforms, Sobel operators, differentials,Prewitt operators, and Roberts Cross detectors, all of which are welldeveloped algorithms, known to those of skill in the art. Adjust fordependent parameters 3004 then modifies the appropriate camera sensorsfor optimal DPR demodulation. In the case of line detection, thiscorresponds to a linear adjustment for the line width lookup table.Determine tones 3203 uses the adjusted line width to determine the DPRtone sent. This process is performed for each region on the image, untilthere are no more regions 3204 remaining. A data structure containingall the regions, with their associated identifiers, is then returned3205.

An additional method for performing DPR demodulation is described inFIG. 33. One or more light sources 101 illuminates a scene 3301. Whenthe image sensor on mobile device 103 acquires a sequence of images3302, the brightness of any given pixel depends on both the details ofthe scene as well as the illumination. In this context, “scene” refersto the area within view of the camera. The scene dependence means thatpixels in the same row of the image will not all have the samebrightness, and the relative brightness of different image rows is notsolely dependent on the modulated illumination 3301. If one were to takethe Fourier transform of such an image, both the frequency content ofthe illumination, as well as the frequency content of the underlyingscene, will be present.

In order to recover the frequency content of the modulated illuminationindependently of the scene, the contribution of the scene may be removedusing a background subtraction algorithm 3303. The “background” is theimage that would result from un-modulated illumination as opposed to theeffects of modulated illumination 3301. Subtracting the background froman image leaves only the effects of illumination modulation. Onepossible implementation of a background subtraction method uses a videosequence. If a video of a scene illuminated with modulated light isrecorded, the light and dark bands may appear at different locations ineach frame. For any modulation frequency that is not an exact multipleof the video frame rate, there will be a resulting beat frequencybetween the video frame frequency and the illumination modulationfrequency. The illumination signal will be in a different part of itsperiod at the beginning of each frame, and the light and dark bands willappear to be shifted between video frames (i.e. the bands will appear tomove up or down across the scene while the video is played). Althoughthis algorithm is described with the use of a video sequence, otherembodiments may perform background subtraction using still images.

Because the bands move between video frames, the average effect of thebands on any individual pixel value will be the same (assuming that in along enough video each pixel is equally likely to be in a light or darkband in any given frame). If all the video frames are averaged, theeffects of the bands (due to the illumination modulation) will bereduced to a constant value applied to each pixel location. If the videois of a motionless scene, this means that averaging the video frameswill remove the effect of the bands and reveal only the underlying scene(plus a constant value due to the averaged bands). This underlying scene(the background) may be subtracted from each frame of the video toremove the effects of the scene and leave only the effects ofillumination modulation 3301.

FIG. 34 contains an implementation of a possible background subtractionalgorithm 3304. A frame buffer 3402 accumulates video frames 3401. Thesize of this buffer can vary, depending on the memory capacity of mobiledevice 103 and the required time to first fix. Frame averaging 3403computes the average based on the frames in the buffer 3402. The averageof these frames is used to generate background frame 2704. Thebackground frame may be acquired using a number of different averagingtechniques 3403, including a simple numerical average, a normalizedaverage (where each frame is divided by the sum of all the frames),Gaussian averaging, or by doing a frame difference between subsequentframes. A frame difference simply subtracts subsequent frames from oneanother on a pixel-by-pixel basis.

For video of a scene with motion, simple averaging of video frames willnot yield the underlying scene background. FIG. 35 describes a techniquefor dealing with motion between frames, which is a likely scenario whendemodulating DPR signals on mobile device 103. Motion compensation 3501is necessary to best determine the underlying scene. By determining themotion between video frames (for example, shifting or rotation of thewhole scene due to camera movement), each video frame may be shifted ortransformed such that it overlies the previous frame as much aspossible. After performing these compensatory transforms on each framein motion compensation 3501, the video frames are averaged 3403 to getthe scene background 3404. Phase correlation is one possible method ofestimating global (i.e., the whole scene moves in the same way, as inthe case of camera motion while recording video) translational motionbetween frames. The 2D Fourier transform of a shifted image will be thesame as that of the original image, except that a phase shift will beintroduced at each point. Normalizing the magnitude of the 2D Fouriertransform and taking the inverse transform yields a 2D image with a peakoffset from the center of the image. The offset of this peak is the sameas the shift of the shifted image. Those skilled in the art willrecognize that additional methods for motion compensation 3501 includeKernel Density Estimators, Mean-shift based estimation, andEigenbackgrounds.

After removing the background scene, Fourier Analysis may be used torecover the DPR tone based on signals received from modulated lightsource 103. Specifics of this method are further described in FIG.36-43. FIG. 36 contains a sample image 3601 of a surface illuminated bya light source undergoing DPR modulation. The image is being recordedfrom a mobile device using a rolling shutter CMOS camera. The stripes3602 on the image are caused by the rolling shutter sampling function,which is modeled in by the sequence of Dirac Combs 2801 in FIG. 28.

FIG. 37 shows the result 3701 of performing background subtraction onthe raw image data from FIG. 36. Background subtraction is used toextract the stripes from the raw image data. The result is an image ofalternating black/white stripes that represents the discrete time-domainrepresentation of the transmitted DPR signal. The stripes 3702 are muchmore pronounced than in the raw image data from FIG. 36 due to theimprovement from background subtraction.

Illumination modulation affects each row of a video frame identically,but imperfect background subtraction may lead to non-identical pixelvalues across image rows. Taking the Fourier transform of row valuesalong different image columns, then, may produce different illuminationsignal frequency content results. Because the true illumination signalfrequency content is the same for the entire image, a technique toreconcile these different results may be employed. One possible methodis to assign the average pixel value for any given row to each pixel inthat row. This method takes into account the information from each pixelin the row, but by yielding uniform row values gives a singleillumination signal frequency content result when taking the Fouriertransform of row values along an image column. FIG. 38 displays theresults of applying row averaging 3801 to the background subtractedimage 3701. The stripes 3802 are much more visible as a result of therow averaging, and they are also more consistent across rows.

FIG. 39 shows the Fourier transform 3901 of the row averaged image 3801from FIG. 38. There is a peak frequency at the DPR tone of 700 Hz, aswell as a DC component at 0 Hz. The peak frequency is used to identifythe sequence of tones, and thus the transmitted identifier.

FIG. 40 shows the Fourier transform 4001 from FIG. 39 after applying ahigh-pass filter. The DC component of the signal is removed, whichallows a peak frequency detector to move to detection of the DPR tonefrequency.

FIG. 41 shows a 2-D Fast Fourier Transform 4101 of the post-processedDPR modulated signal data 3701. In comparison to the 1-D Fourieranalysis performed in FIGS. 38-40, 2-D Fourier analysis of the DPRmodulated signal 3601 may also be performed. 2-D Fourier Analysis is apopular and widely used technique for image analysis. Because there area number of software libraries that are highly optimized for performingmultidimensional FFTs, including OpenCV, multidimensional Fourieranalysis is a viable alternative to the 1-D analysis. The DPR tones 4102may be easily seen across the vertical axis 4103 of the 2-D FFT.Brighter areas on the FFT image 4101 correspond to areas on the imagewith higher spectral content. A peak may be seen at the origin 4104,which corresponds to the DC component of the DPR signal.

FIG. 42 shows a low-pass filtered version 4201 of the 2-D FFT 4101. Thefiltered image 4201 contains dark areas 3502 at the higher frequencieson the image. The low pass filter rejects the higher frequencies. Thisis a key component of successful DPR demodulation. As discussedpreviously, DPR modulation relies on transmitting digital signals atdifferent frequencies. When using Fourier analysis on these signals,higher frequency harmonics appear, in particular at higher duty cycles.These higher frequency components act as noise in the signal, soremoving them with filtered image 4201 is one technique for recoveringthe transmitted tones.

When performing spectral analysis in the case of a 1-D FFT 3901 in FIG.39, it was necessary to remove the DC component of the DPR signal. PWMsignals 1901-1903 will contain a significant DC component, which needsto be filtered before moving on to extract the transmitted DPR tone.FIG. 43 shows a high-pass filtered version 4301 of the 2-D FFT 4101. Thedark area 4302 at DC demonstrates the result of the high-pass filter,which rejects the DC noise component. The higher frequency bands 4303are still contained in the signal, allowing the demodulator to determinethe peak frequency.

A source of spectral noise in many digital images is the occurrence ofregular brightness patterns. Such patterns are commonly produced byclothing designs, structural surfaces (e.g., brick walls, tile floors,ceiling tiles), carpeting designs, and other objects. Regular patternstend to produce peaks in image FFTs and may thus confound the detectionand identification of peaks corresponding to DPR signals in images asdescribed in an illustrative fashion hereinabove. False positives (i.e.,erroneous detections of DPR tones that are not present) and falsenegatives (i.e., failures to detect DPR tones that are present) may bothbe caused by spectral noise from visual patterns.

The following techniques are contemplated for mitigating the effect ofspatial patterns in various embodiments of the present invention. Mobiledevices typically focus their cameras automatically, but in some mobiledevices it is possible to defocus the camera under software control(e.g., under the control of a mobile app). Such defocusing may beachieved simply by commanding focus at the nearest possible distance, onthe presumption that the mobile device is unlikely to at closest-focusrange from a wall, floor, or other patterned surface. A user may beinstructed by software on their mobile device to point the device'scamera at a surface at least 3-4 feet distant (e.g., to hold the unitapproximately level at waist height so that the camera is pointing atthe floor), increasing the likelihood that a closest-focus image will bedefocused. In another embodiment, defocusing employs an adaptivealgorithm that seeks maximum defocus, e.g., by seeking a lens positionthat minimizes image contrast. This technique inverts themaximum-contrast autofocus method deployed in many digital imagingsystems (which seeks a lens position that maximizes, rather thanminimizes, image contrast).

As is well known, defocusing has the effect of low-pass filtering animage. That is, brightness changes that vary slowly across an image tendto be preserved with defocusing while brightness changes that varyrapidly tend to be attenuated. The precise equivalent filtercharacteristics of defocusing depend on distance of the camera fromvarious surfaces in view, degree of defocusing, lens characteristics,and other factors, and so cannot be precisely defined or controlled forpurposes of DPR-modulated light signal detection. However, a significantdegree of low-pass filtering is usually obtainable by defocusing and islikely to aid FFT peak detection of DPR tones.

Defocusing does not affect the DPR modulated light signal component ofthe defocused image (e.g., the stripes in FIG. 36), because the stripesproduced on the image by the DPR signal are never part of the sceneimaged by the camera lens; they are a purely digital artifact producedby the phase relationship of the rolling shutter exposure mechanism tothe DPR modulated light signal. Defocusing therefore has no tendency tofilter the DPR signal regardless of the DPR signal's frequencycharacteristics.

Alternatively or additionally to optical defocusing prior to imagedigitization, digital filtering after image digitization may beperformed by software (e.g., by an app running on the mobile device),according to various embodiments, to mitigate the effects of spectralnoise from visual patterns. Digital low-pass filtering, as will be clearto persons familiar with the art of digital signal processing, consistsessentially of the performance of mathematical operations on numbers(e.g., pixel brightness values) which may represent samples of a signal(e.g., an optical image). However, digital filtering cannot substitutedirectly for the low-pass filtering effect of defocusing because digitalfiltering operates on the digital image itself, including any artifactsthe digital image may contain (e.g., DPR striping). Digitalfiltering—especially simple filtering—therefore tends to affect the DPRcomponent of an image along with any patterns arising from the opticalimage. Nevertheless, in various embodiments, digital filtering and otherforms of digital signal processing (e.g., background subtraction) arecontemplated, alternatively or additionally to defocusing, to enhanceDPR signal detection in the presence of irrelevant image patterns.

In various embodiments, after an image has been low-pass filtered byoptical defocusing prior to digitization, and/or by digital low-passfiltering after digitization, and/or possibly to other forms of signalprocessing, the filtered digital image is subjected to FFT calculationand attempted peak frequency detection of any DPR tone frequency orfrequencies present in the image as described hereinabove.

FIG. 44A shows a portion of an illustrative image of a masonry wall infocus. Fine horizontal line structures 4402, 4404 are apparent in theimage and recur periodically throughout the image. FIG. 44 B shows thesame portion of the illustrative image after deliberate optical focusingat the shortest possible range (i.e., “manual” defocusing). Broad,partially low-frequency bands of brightness and darkness 4406, 4408 areapparent but no fine image structures are visible such as the structures4402, 4404 in FIG. 44A. The broad, horizontal brightness bands 4406,4408 are a DPR tone frequency artifact and the object of DPR tonedetection in processing this scene. The broad brightness bands 4406,4408 may be seen in FIG. 44A as well, aligned by chance with alternatemasonry rows.

FIG. 45 shows FFTs of the full images from which the partial images ofFIG. 44A and FIG. 44B are taken. The FFT 4502 (dashed line) of thefocused image features prominent peaks at around 650 Hz, 1450 Hz, and2900 Hz. The peak at ˜650 Hz corresponds to the DPR tone frequencypattern seen in FIG. 44B. The peaks at ˜1450 Hz and ˜2900 Hz arise fromthe fine line structures seen in FIG. 44A and may cause a DPR-seekingalgorithm to produce false positives. The FFT 4500 (solid line) of thedefocused image preserves the true DPR peak at ˜650 Hz but reduces thespurious peak at ˜1450 Hz and displays no spurious peak at ˜2900 Hz. ADPR-seeking algorithm is therefore more likely to produce accurateresults using the FFT 4500 of the defocused image than using the FFT4502 of the focused image. Note that for low frequencies, the amplitudesof the focused FFT 4502 and the defocused FFT 4500 are indistinguishableon the scale of FIG. 45; only above ˜1000 Hz is the filtering effect ofdefocusing apparent. In FIG. 45, the unit Hertz (Hz, one cycle persecond) is used even though the signals in question are spatial, nottemporal; Hz is here used as a proxy unit for spatial frequency.

Novel Techniques for Updating Location Estimates for a Mobile Device

A mobile device employing DPR modulated light signals to estimate itslocation may, in some states of operation, present the mobile device'suser with a graphic interface that includes or consists essentially of amap. The map may be oriented on the display of the mobile device on thepresumption that the user typically orients the device perpendicularlyto the plane of the user's body. The map may also feature a “you arehere” cursor that visually identifies the user's location (i.e., thelocation of the mobile device, presumed to be co-located with the user).The user interface may thus present the user with spatial mapinformation about the layout of the user's surroundings (e.g., aisles,walls, doors, displays, kiosks), “you are here” locational information,and directional (heading) information.

In the operation of such a user interface, it is in general desirablethat all information presented, including information about deviceposition and orientation, be as accurate as possible and be presented tothe user in a manner that is as clear, useful, and pleasant to view aspossible. In partial fulfillment of these goals, various embodimentsemploy a variety of methods to calculate and display an estimate of theuser's position that is updated in real time as the mobile deviceopportunistically identifies DPR modulated signals from lights invarious positions.

As described hereinabove, in various embodiments software on the mobiledevice and/or on a back end or server, employing data from alight-sensing device of the mobile device, cyclically seeks to identifylight identification codes in ambient light. If no such codes are found,then the location of the mobile device cannot be estimated frominformation about the location of coded LED light sources. If the IDcode of at least one LED light source is found, then the mobile device'sposition may be estimated.

A first illustrative method of estimating the position of a mobiledevice by an app running on the device in a space containingDPR-modulated LED light sources is as follows. When the app detects thepresence of one or more light ID codes by analyzing data from alight-sensing device of the mobile device, the location of the detectedone or more LEDs may be obtained from a server as shown in FIG. 7, FIG.9, and FIG. 10. An initial estimate of device position may be determinedfrom one or more light ID code detections according to various methods;most simply, an initial estimate may be given by the location of thefirst LED whose ID code is detected. Once an initial position estimateis available, a portion of a map, which may also be obtained from adatabase on a server as shown in FIG. 7, FIG. 9, and FIG. 11, may bedisplayed on the device with the device's location indicated on the mapby a cursor. A raw value for the orientation of the device may beobtained from a magnetic compass built into the device and, as shall bemade clear in figures and accompanying description hereinbelow, may berectified or corrected using field-map information contained in the Mapsdatabase in the server (FIG. 9 and FIG. 11). The map may be oriented onthe display screen of the mobile device using raw or correctedorientation information, incorporating the assumption that the usertypically holds the device perpendicularly to the plane of their body.The user thus is presented with an initial estimate of their positionand orientation in the context of a spatial area map or portion thereof.The extent of the map portion displayed may be settable by the user:e.g., a larger or smaller portion of the area map surrounding the user'slocation may be displayed in response to user commands such astouchscreen pinch gestures.

As time goes on, detections of one, two, or more light IDs may occur.Also, multiple detections of the IDs of one or more particular lightsmay occur. Even if the user is stationary, IDs of multiple lights may bedetected, and if the user moves about sufficiently it is likely thatthey will move from detection range of one or more lights to withindetection range of one or more other lights, and that ongoing IDdetections by the mobile device will reflect such changes.

A first method of calculating a time-varying estimate of device positionusing a time series of ID detections, herein termed the Static Method,is designed to produce highly confident location estimates according tovarious embodiments of the invention. The Static Method considerswhether p percent or more of IDs from a single light have been detectedin the last n ID detections. The percent threshold parameter p andlookback time parameter n may be specified by the designers of thesystem that implements the Static Method, or may be settable by thedevice user or by software running on a different computer (e.g., theserver). Parameter setting may also be performed by software in anadaptive (time-varying) manner.

In the Static Method, if p percent of the last n IDs detected belong toa single light (Light A), then the current location estimate is set tothe location of Light A. As the user moves from the vicinity of Light Ato the vicinity of another DPR modulated light, Light B, ID detectionswill shift, suddenly or gradually, from detections solely or primarily(i.e., more than p percent) of Light A to detections solely or primarilyof Light B. When the criteria for light-source identification (i.e., ppercent or more of the last n detections) are met for light B, thelocation estimate will be updated to the location of Light B.

The Static Method only supplies position estimates that correspond tothe positions of individual light sources. For example, a Static Methodposition estimate cannot be intermediate between the locations of aLight A and a Light B, or at the center of a triangle whose vertices areLight A, Light B, and a Light C.

An advantage of the Static Method is that it is likely to discard falsepositives. That is, the Static Method is very unlikely to estimate thelocation of the mobile device as being the location of any light sourcethat is not the light source nearest to the device. However, the StaticMethod has at least four notable disadvantages, herein termed Lag, Snap,Bounce, and Failure to Estimate:

-   -   1) Lag. As a user moves with their mobile device from the        vicinity of one light source to another, there is a generally        noticeable lag, on the order of n times the duration of a single        detection cycle, in the updating of the position estimate        displayed on the mobile device. That is, even after a user has        approached a Light B, the location estimate displayed on their        device may still show the user as located at a Light A for a        noticeable length of time.    -   2) Snap. Because position estimates can coincide only with        light-source locations, they change suddenly (snap) from one        light-source location to another. The depiction on a mobile        device map of this snapping from one light source to another can        be jarring or disconcerting to users. Users may even have the        perception that the app is “not working,” since their own        movements through the space are continuous but the depiction of        their changing position is not. Measures to perceptually        mitigate snap, such as animating smooth movement of the “you are        here” cursor to each new position estimate, delay the portrayal        of position updates and therefore tend to worsen lag.    -   3) Bounce. When the mobile device is between two discrete        detection points, e.g. approximately halfway between a Light A        and a Light B, there is a tendency for the location estimate to        oscillate or bounce between Light A and Light B. This is        disconcerting for users. Measures to prevent bounce, such as        requiring position estimates to remain stable for some period of        time before updating the device display, tend to worsen lag.    -   4) Failure to Estimate. Fixed location-estimation criteria may        never be met by a device that is not receiving signals of        sufficient quality: no location estimate is then offered, or an        existing estimate is never updated. Or, even if the device is        successfully identifying light IDs based on high-quality        signals, the criteria for position estimation may never be        fulfilled: for example, if a device is equally illuminated by        four LEDs with p set at 30%, the percent of ID detections        attributed to each of the four LEDs over a moving window of n        detections may hover near 25% and never exceed the p threshold.        Yet it should be possible to estimate a device's position from        so many valid ID detections.

A second method in various embodiments of calculating a time-varyingestimate of device position using a time series of ID detections, hereintermed the Statistical or Continuous Method, is designed to producelocation estimates that are not susceptible to the drawbacks of theStatic Method. Given that an initial position estimate has been producedby some means (e.g., as the location of the first light source to beidentified), the Statistical Method updates the position estimate everytime a light source ID is detected. There are no threshold criteria asin the Static Method, and estimated positions are not restricted to thelocations of LED light sources. Rather, the Statistical Method moves theposition estimate fractionally or incrementally in the direction of eachlight source that is detected (or does not change the position estimateif the latest light source detected is co-located with the currentposition estimate). The size of each incremental movement may beweighted according to various criteria, e.g., by how long it has beensince the previous successful ID detection (e.g., if it has been longersince the previous successful ID detection, then the position estimateis moved farther). A position estimate from the Statistical Method maystabilize (or approximately stabilize) near positions that areco-located with LED light sources, or that are between two LED lightsources, or that are in the midst of three or more LED light sources.

A basic, settable parameter of the Statistical Method is herein termedthe “max lag time.” Max lag time is essentially the time that theStatistical Method will take to update completely from the position of aLight A to the position of a light B if the device is movedinstantaneously from one to the other, where the distance between LightA and Light B is sufficient to prevent ID detections of each light atthe location of the other. In the event of such a hypothetical jump,detections of Light A would suddenly cease and detections of Light Bwould suddenly begin. If the mobile device is capable of performing Kdetections per second and the max lag time is M seconds, then theinitial position estimate (i.e., the position of Light A) will shift M/Kof the way toward the position of Light B upon each of the first Kdetections after the jump and thereafter will coincide with the positionof Light B. In other words, immediately after the hypothetical jump theposition estimate would make K shifts, moving M/K of the way from LightA to Light B on each shift and being coincident with the position ofLight B after the Kth shift, which occurs at the end of the max lag timeof M seconds. For example, for a max lag time of 1 second, a devicecapable of 5 detections per second, after hypothetically jumping fromLight A to Light B, would make 5 position adjustments in 1 second, eachshifting the position estimate ⅕ of the way from Light A to Light B.Since the maximum number of ID detection cycles per second tends to befixed for a given device, specification of the max lag time has theeffect of controlling how quickly the Statistical Method tracks changesin device position.

The Statistical Method is so called because the position estimate itproduces is in effect a moving average of weighted position vectors. Thewindow may be finite in length, i.e., light ID detections older thansome threshold specified in seconds or number of ID detections may bedisregarded by the method. The Statistical Method has the drawback thatfalse positives (incorrect ID detections) will be incorporated into theposition estimate as they occur, potentially causing the positionestimate to jitter or wander. However, the Statistical Method overcomesthe four drawbacks of the Static Method described hereinabove:

-   -   1) Lag. Lag is greatly diminished by the Statistical Method. The        “you are here” cursor viewed by a user begins to move        immediately after detection of any light ID that is not        co-located with the current position estimate.    -   2) Snap. Direct display on a user map of incremental changes to        the position estimate may be perceived as a series of small        jumps or positional snaps; however, depiction of incremental        updates to cursor position may be smoothed by animation while        increasing lag only slightly. With or without smoothing        animation, the behavior of the Statistical Method position        estimate more closely approximates typical user expectations        (i.e., continuous movements in space are reflected in        approximately continuous movements of a “you are here” cursor).    -   3) Bounce. Locations intermediate between light-source locations        may be estimated by the Statistical Method: i.e., a position        somewhere between two light sources (if both sources are strong        enough at the device's position to be detected at least        intermittently). In intermediate locations (e.g., halfway        between a Light A and a Light B), a slight oscillation of the        position cursor may be observed by a user as ID detections occur        first for one light and then for the other, but disconcerting        bouncing or snapping of the cursor between the locations of        Light A and light B will not be observed.    -   4) Failure to estimate. The Statistical Method can only fail to        estimate if no light ID detections occur at all.

FIG. 46 is a high-level flow chart of an illustrative version of theStatic Method for mobile device position estimation according to variousembodiments of the invention. The method, which is cyclic, may beentered at Acquire Camera Frames 4600 and exited from any part of thecycle by software interrupt (quitting the program). At Acquire CameraFrames 4600, the mobile device acquires a digital photograph. (Invarious other embodiments, a series of light-intensity readings may beobtained from a non-imaging light sensor of the mobile device.) Thephotograph, stored in the storage system of the device (storage 504 inFIG. 5), is subjected to signal processing (Signal Processing 4602) forID detection in the CPU of the mobile device (CPU 502 of FIG. 5). Signalprocessing in block 4602 may, for example, comprise backgroundsubtraction processing such as is shown and described in U.S. Pat. No.8,520,065, the entire contents of which are incorporated herein byreference. The app software of the mobile device waits for yes/noconfirmation or stabilization of light-source ID identification (LightSource ID Confirm? 4604) according to the criteria described hereinabove(p percent of detections the last n detections). If no light ID isconfirmed, the device returns to Acquire Camera Frames 4600. If fewerthan n detections have occurred, additional detections will be sought atleast until n detections have occurred, and thereafter until p percentof the latest n detections are of a single light source. If a lightsource ID is confirmed, the app updates its position estimate (UpdatePosition Estimate 4606) to the position of the confirmed light source.

FIG. 47 is a high-level flow chart of an illustrative version of theStatistical Method for mobile device position estimation according tovarious embodiments of the invention. The method, which is cyclic, maybe entered at Acquire Camera Frames 4700 and exited from any part of thecycle by software interrupt (quitting the program). At Acquire CameraFrames 4700, the mobile device acquires a digital photograph or seriesof readings may be obtained from a non-imaging light sensor. These data,stored in the storage system of the device (storage 504 in FIG. 5), aresubjected to signal processing (Signal Processing 4702) for ID detectionin the CPU of the mobile device (CPU 502 of FIG. 5). Remarks regardingSignal Processing 4602 in FIG. 46 apply equally to Signal Processing4702. If a light source ID is identified, the app incrementally shiftsits position estimate (Update Position Estimate 4704) in the directionof the detected light source as described hereinabove.

The Statistical Method cycle of FIG. 47 in general entails a lowercomputational burden than does the Static Method cycle of FIG. 46, asthere is no step devoted to the examination of location confirmationcriteria. That is, the Statistical Method tends to be morecomputationally efficient than the Static Method.

FIG. 48 is an illustration of aspects of (a) an illustrative physicalspace (represented by a rectangle) in which a mobile device moves andwhich contains two ID-broadcasting light sources (column 1, ActualPosition 4800) and (b) illustrative device displays (column 2, OutputDisplay Static Method 4802; also column 3, Output Display StatisticalMethod 4804) that indicate the estimated position of the deviceaccording to either the Static Method or the Continuous Method. Actualposition, Static Method output display of position estimate, andStatistical Method output display of position estimate are shown intop-down view for four consecutive times T1, T2, T3, and T4. The fourtimes may not be evenly spaced.

In column 1, Actual Position 4800 at time T1, a mobile device physicalposition 4806 (indicated by an X) within a space 4808 is directly undera first light source 4810 (indicated by a dashed circle) broadcasting adistinct ID. A second light source 4812 is also present in the space4408. For clarity, device physical position 4806 and other repeatedelements of FIG. 48 are explicitly labeled in FIG. 48 only for time T1.In column 1, Actual Position 4800 at time T2, the physical deviceposition 4806 is directly under LED light 4812. From T1 to T2, the userof the device has moved normally from a point under the first lightsource 4810 to a point under the second light source 4812. For times T2,T3, and T4, as depicted in the rest of column 1, Actual Position 4800,the device remains under the second light source 4812.

In column 2, Output Display Static Method 4802, the physical space 4808is schematically depicted as it might be on a user's device display.That is, a representation 4820 of the physical space 4808, arepresentation 4814 of the device position estimate (indicated by acaret), and representations 4816, 4818 of the two light sources 4810,4812 are all shown in the display. At time T1, the position estimate4814 is coincident with the symbol 4814 for the first LED light source4810. By time T2 the user is physically under the second light source4812, but the position indicator 4814 is still shown in Output DisplayStatic Method 4802 as coincident with the first light symbol 4816. Evenat time T3 the display has not changed from its state at T1. By time T4,the Static Method criteria for location confirmation have been met andthe position indicator 4814 has moved suddenly to be coincident with thesecond light symbol 4818. The incorrect, unchanged position display ofOutput Display Static Method 4802 for times T2 and T3 constitutes theundesirable “lag” described hereinabove.

In column 3, Output Display Statistical Method 4804, the same displaysymbols and conventions are used as in column 2, Output Display StaticMethod 4802. At time T1, the position estimate 4814 is coincident withthe symbol 4814 for the first LED light source 4810. By time T2, theuser is physically under the second light source 4812. At some timebetween time T1 and time T2, the device 4806 began to detect the ID ofthe second light 4812; consequently, by time T3 the position indicator4814 has been incrementally adjusted, perhaps repeatedly, in thedirection of the second light symbol 4818. Similarly, by time T3 theposition indicator 4814 has been further incrementally adjusted in thedirection of the second light symbol 4818. By time T4, the positionindicator 4814 is coincident with the first second symbol 4818.

The reduction of lag for the Statistical Method as compared to theStatic Method is evident in the movement of the position indicator 4814for times T2 and T3. The incremental movement of the Statistical Methodposition indicator for times T2, T3, and T4 indicates the typical,approximately smooth movement of position indicator for the StatisticalMethod, as opposed to the sudden jump of the position indicator fromtimes T3 to T4 for the Static Method. Finally, the ability of theStatistical Method to estimate locations intermediate between lightsources is apparent in the display of the position indicator 4814 fortimes T2 and T3.

It will be apparent to persons of ordinary skill in the science ofcommunications and signal processing that both the Statistical andStatic Methods employ the relative frequency of detections of signalsfrom given sources as a proxy for signal strength: that is, strongersignals are likely to be more frequently detected than relatively weaksignals. Therefore, in various embodiments, other methods of employingrelative signal strength or received signal strength indication (RSSI)of light sources as a basis for updating update the position estimatesof the Statistical and Static Methods, or to produce such estimates byother methods, are contemplated and within the scope of the invention.For example, when peak detection is applied to FFTs of light-intensitydata derived from optical sensors, the relative heights of detectedpeaks may be used to directly estimate relative RSSI and thus therelative distances of distinct light sources. Such direct RSSIestimation may be used alternatively or additionally to frequency of IDdetection (indirect RSSI estimation) to update device positionestimates.

Techniques for Presenting Accurate Orientation and Location Informationon a Device Display

FIG. 49 is a schematic, top-down, illustrative representation of theeffect of local magnetic field anomalies on heading measurements made bya mobile device operating in the context of an indoor position locationsystem (as in, e.g., FIG. 7). In FIG. 49, a mobile device 4900 (e.g.,smartphone) is being carried by a user (not shown) along anapproximately straight path 4902 between barriers 4904, 4906 (e.g.,shelving rows) that define an aisle in the space between them. TwoID-broadcasting LED light fixtures 4908, 4910 (dashed circles) arelocated above the aisle. The horizontal component of the magnetic field4912 of the Earth, which is approximately uniform on the scale depictedand is here idealized as pointing to true North (indicated by a compassrose 4914), is represented as a set of parallel dashed arrows diagonallytraversing the figure. Due to the presence of metallic masses,electrical machines, or other causes, the magnetic field at variouspoints in the space depicted deviates from the Earth field 4912. Forexample, at the location of the first light 4908, the actual field 4916(bold arrow) deviates by angle A1 from the Earth field, and at thelocation of the second light 4910, the actual field 4918 (bold arrow)deviates by angle A2 from the Earth field. Angles A1 and A2 are denotedby curved arrows bracketing each angle. (The local field may vary instrength, as well as in direction, from the unperturbed Earth field, butvariations in field strength will typically not affect headingmeasurements unless the field strength is reduced to a level notreliably detectable.)

In an illustrative operating scenario, the device 4900 is carried to thelocation of light 4908 and detects the broadcast ID of the light 4908.The device, which is presumptively held in a significantly non-verticalposition and perpendicular to the plane of the user's body, wirelesslyqueries a server for the location of light 4908; the server consults itsdatabase and tells the device what location estimate to use. The serveralso transfers map information to the device, which the device displaysas a map for the benefit of its user, with “you are here” positionindicator. Moreover, the device takes a heading measurement using itsinternal compass and orients the map accordingly.

However, at the position of first light 4908, the mobile device 4900measures a field that deviates from the Earth field by angle A1.Therefore, if the map display is oriented using the raw compass readingmeasurement, it will be misaligned with the physical environment byangle A1, which will tend to degrade the performance of the indoorpositioning system.

In various embodiments, the problem of local misalignment is addressedby the following technique. A commissioning process, to be made clear insubsequent figures and accompanying explanatory text, stocks the serverMaps database with a layer of local field readings, herein termed theDeviation Table. The Deviation Table records the deviation of the localfield from the unperturbed Earth field at some set of points coveringpart or all of the space served by the indoor positioning system (e.g.,the locations of all ID-broadcasting LED lights in the space). FIG. 50Aand FIG. 50B schematically depict the information thus recorded for twoillustrative locations. At a first point 5000 (e.g., a location similarto that of light 4908 in FIG. 49), the unperturbed Earth field 4912(here idealized as pointing due North as indicated by compass rose 5002)is at angle θ_(M) with respect to an arbitrary reference axis 5004 andthe deviant, actual field 5006 is at angle θ_(D1). As shown in FIG. 50B,at a second point 5008 (e.g., a location similar to that of light 4910in FIG. 49), the unperturbed Earth field 4912 is also at angle θ_(M) andthe deviant, actual field 5010 is at angle θ_(D2). When a device isestimated to be at point 5000, the local deviation angle θ_(D1) is addedto the angle θ_(M) of the unperturbed Earth field to produce a correctedheading; when a device is estimated to be at point 5008, the localdeviation angle θ_(D2) is added to the angle θ_(M) of the unperturbedEarth field to produce a corrected heading. Such addition may beperformed either by the server, which transmits the corrected heading tothe mobile device, or by software running on the mobile device itself.

FIG. 51A and FIG. 51B depict aspects of the illustrative mobile device4900 in two possible states of operation when at the position of firstlight 4908 in FIG. 49. In FIG. 51A, the screen 5100 of the device 4900is displaying a map oriented according to a raw compass measurement. Thedevice 5100 is aligned with the direction of motion 4902 in FIG. 49(along the aisle), but the map (which includes depictions 5012 of aislebarriers and a “you are here” cursor 5014) is rotated erroneously by anangle of A1. In FIG. 51B, the map displayed on the screen 5100 of thedevice 4900 is oriented accurately using a corrected heading.

As will be clear from foregoing figures and descriptions, the estimatedposition of a mobile device in an indoor positioning system will notalways coincide exactly with its physical position. Thus, some degree ofresidual misalignment—smaller when the device is physically closer toits estimated position, larger when the device is physically fartherfrom its estimated position—may occur even after the application of acorrection from the Deviation Table.

In the heading correction method described above, heading corrections ordeviations are recorded at a set of points corresponding to light-sourcelocations. In embodiments which allow for the estimation of position atpoints intermediate between light-source locations (e.g., in thoseemploying the Statistical Method of position estimation), headingcorrections for points not directly under light sources may becalculated by any of several well-known interpolation methods eitherprior to system operation or in real time, as intermediate locationestimates occur. For example, at a given intermediate location, themeasured deviation values for nearby light-source locations may beweighted by the inverse of their distances from the intermediatelocation (closer points, heavier weighting) and averaged to produce anestimate of the deviation at the intermediate location. Alternatively, aversion of the Statistical Method for position estimation may be used toupdate orientation correction estimates. Interpolative estimations ofdeviation corrections may have two benefits. First, it may minimize thedeviation error for mobile devices not physically located directlybeneath light sources. Second, it may prevent sudden jumps or snaps ofmap orientation: as the “you are here” cursor moves more or lesssmoothly through the map space, the orientation of the map adjusts moreor less smoothly as well. Orientation lag and snap may thus both beavoided or mitigated.

FIG. 52 is a high-level flow chart of an illustrative method forapplying deviation corrections to headings measured by a mobile devicein an indoor positioning system according to various embodiments of theinvention. After starting, the algorithm is cyclic and may be exited atany point by software interrupt (quitting the program). It is presumedin FIG. 52 that the device is already, by the time of Start,successfully detecting light-source IDs and that a position has beenestimated for the device. First, the heading correction of the mobiledevice is initialized to zero (block 5200). Next, a raw headingmeasurement is obtained from the device's internal compass (block 5202).The device queries the server for the correction value for the currentlocation (block 5204). In various embodiments, compass headingmeasurement (block 5202) and correction value querying of the server(block 5204) may occur concurrently, rather than sequentially asdepicted in FIG. 52. After the device has received the headingcorrection for the current position estimate from the server and updatedthe heading estimate (block 5206), the map displayed to the user isrotated using the updated heading estimate to be consistent with theuser's physical orientation.

As noted above, it is preferable for the Deviations Table in the serverMaps database to be calibrated or commissioned with deviationmeasurements for a set of locations in the space to be served by theindoor positioning system (e.g., the locations of the LED light sourcesin the system). FIG. 53 is a high-level flow chart of an illustrativemethod for populating a Deviations Table. After starting, the algorithmis cyclic and may be exited at any point by software interrupt (quittingthe program). It is presumed for the illustrative method of FIG. 53 thatthe space of interest may be mostly or wholly covered by a set ofstraight-line traverses (e.g., along aisles), but in various otherembodiments curving paths may be accommodated. It is also presumed thata correctly oriented map of the space in question has already beencreated and stored in the server Maps database and that the trueorientation of the map is known (e.g., by surveying techniques).

First, the server's heading correction table (Deviation Table) for thespace to be calibrated is initialized to zero (block 5300). Next, anassistant carrying an appropriately equipped mobile device walks along acalibration path (e.g., from one side of a retail space to the other)holding the mobile device in a significantly non-vertical orientationand parallel to their direction of travel, and the device scans forlight source IDs in the ambient light (block 5302). If a light source isnot identified (branch point 5304), the assistant continues to walk andthe mobile device continues to scan for light source IDs (return toblock 5302). If a Light A is identified with sufficient consistency toallow an estimate of the device's position (as, e.g., collocated withLight A) the device takes a heading measurement and compares it to thetrue heading of the device (block 1506). The true orientation (directionof travel) of the device may be inferred from the layout of the spatialmap, whose true orientation is, as noted, already known: for example, ifthe assistant is walking down an Aisle Z, orienting the mobile devicealong their direction of travel, then the true orientation of the mobiledevice may be inferred as being the pre-measured orientation of Aisle Z.Once the device has calculated the deviation between the measuredheading and the true heading at its current location, the device reportsthat deviation to the back end or server, which associates thatdeviation with that location in a stored table (the Deviation Table)(block 1508). If the assistant does not signal that they have reachedthe end of the calibration path (branch point 1510), then the assistantcontinues walking and the device continues to scan for light sources(return to block 5302). If the assistant does signal that they havereached the end of the calibration path (branch point 5310), then theserver checks to see if its Deviation Table for the space in question isfull (i.e., if the last calibration path has been traversed) (branchpoint 5312). If the last calibration path has not been walked, thealgorithm waits for the assistant to signal that they are ready to begintraversing the next calibration path (block 5314). When the assistantdoes signal their readiness to begin traversing the next calibrationpath, the algorithm returns to block 5302 (assistant begins to walk thenext calibration path). Details of the illustrative procedure herespecified may be varied in various embodiments without significantlyaltering the nature of the calibration procedure, and all suchvariations are contemplated and within the scope of the invention.

It is possible that the entries in the Deviation Table as measuredduring a calibration procedure such as that described with reference toFIG. 53 may gradually lose accuracy as perturbers of the Earth magneticfield are removed from, added to, or reoriented or repositioned withinthe mapped space. One method of compensating for such changes is toperiodically repeat a calibration procedure such as that described withreference to FIG. 53. Another is the method herein termed the ContinuousCalibration Method, which may be employed when the indoor locationsystem in question is in some degree of use. The Continuous CalibrationMethod detects motion and probable device orientation for users of thesystem (e.g., shoppers in a retail space) and uses that information toupdate its Deviation Table in a continuous or ongoing manner.

FIG. 54 is a high-level flow chart of an illustrative version of theContinuous Calibration Method according to various embodiments of theinvention. It is presumed that an indoor positioning system is operablein the space in question, and that at least some users (e.g., shoppers)are moving about the space holding mobile devices and running an app,herein termed the “navigation app,” that allows their mobile devices toperform the light ID detection and other navigation functions describedherein that are necessary for the implementation of the ContinuousCalibration Method as described. After starting, the algorithm is cyclicand may be exited at any point by software interrupt (quitting theprogram).

First, the navigation app detects, by noting two or more distinctlocation estimates in series, that the user is in motion at a consistentspeed and in a consistent direction (block 5400). Here, “consistent”means within some specified range of variation (e.g., the user's speedneed not be perfectly constant). Concurrently, the navigation appproceeds to (a) measure a raw compass heading and to solicit a headingcollection from the back end for its latest or current location estimate(block 5402) and to (b) calculate the likely orientation of the devicefrom light position detections and map layout (block 5404). For example,if a series of position estimates shows that the user is likely movingin a straight line down an Aisle Z, the orientation of the device maywith reasonable probability be conjectured to be in the user's directionof travel and parallel to the run of Aisle Z. Next, the navigation appcompares the corrected compass heading obtained in block 5402 with thecalculated device heading obtained in block 5404, and reports theapparent deviation between the two headings to the back end (block5408). The back end then incorporates this apparent deviation for thecurrent location of the device into its Deviation Table by one of avariety of calculational methods (e.g., by maintaining the deviationrecorded in the Deviation Table as a running average of reportedapparent deviations). Although some error will tend to be associatedwith each apparent deviation, as users may hold their mobile devicesacross a range of angles as they move, such errors are likely to averageout over time, enabling the Continuous Calibration method tosuccessfully compensate for changes in local deviations from the Earthfield over time. After updating its Deviation Table entry, the back endchecks (or the navigation app reports) whether the user continues to bein motion with a consistent speed and direction (decision point 5410);if Yes, the algorithm returns to blocks 5402 and 5404. If No, thealgorithm ends until the same user, or another user, is detected inmotion with a consistent speed and direction, upon which the algorithmrestarts.

In various embodiments, a calibration process such as the illustrativeprocess described with reference to FIG. 53 may be dispensed with, and aDeviation Table may be derived entirely by the Continuous Calibrationmethod described with reference to FIG. 54.

In various embodiments, another technique, herein termed“fingerprinting,” may be employed additionally or alternatively to theother techniques described herein for providing accurate orientation andlocation information to a mobile device user. In fingerprinting, acalibration procedure is used to map the light signals producedthroughout a working space (e.g., retail store interior) by alight-based positioning system. The calibration procedure may employwalk-through techniques similar to the calibration procedure describedherein with reference to FIGS. 52-54, or other means of obtaining asufficiently closely spaced set of measurements characterizing the lightsignal pattern or functional field within the working space: in variousembodiments, airborne devices (small drones) quarter a space, eitherautonomously or under remote control, and acquire measurements that areeither stored on board or transmitted to a back end. Measurement datamay include imagery, overall brightness, signal detections, orientationof the measuring device, and other data. The grid or mesh ofmeasurements so obtained may be stored in a fingerprint database. Insuch embodiments, mobile devices (or computers with which the mobiledevices are in communication) may compare local light-field measurementswith the fingerprint database. Single measurements or series ofmeasurements may be compared to characterize, or to improve thecharacterization of, the location, motion, and orientation of the mobiledevice.

Moreover, as will be clear to persons familiar with the science ofinertial navigation, a mobile device may estimate changes in itsposition, velocity, and orientation by measuring accelerations to whichthe device is subjected and calculating the device's spatial movementstherefrom according to the principles of kinematics. Thus, given aninitial estimate of position and velocity, suitable accelerationmeasurements may enable a mobile device to maintain an updated estimateof the device's position and orientation, although the accuracy of theestimate will tend to decrease over time after the initial estimate.Modern mobile devices often contain accelerometers(acceleration-measuring devices). The employment, in variousembodiments, of inertial navigation techniques based on data fromaccelerometers of mobile devices, additionally or alternatively to theother illustrative methods of updating position and orientationestimates described hereinabove, is contemplated and within the scope ofthe invention.

Frequency Sweep Techniques to Compensate for Unpredictable CameraExposure

In various embodiments, a light source in an indoor positioning systemsignals its identity, or transmits information through pulse widthmodulated DPR as described hereinabove, in the form of a periodic orquasi-periodic variation in brightness (e.g., a square-wave variation inbrightness, as in FIGS. 19A-C, or a sinusoidal variation in brightness).The frequency of a brightness variation may be identified by searchingfor a dominant peak in a spectrum (e.g., FFT) that is derived from oneor more digital images or some other series of ambient lightmeasurements: the frequency at which a strong peak is found (e.g.,approximately 650 Hz in FIG. 45) is, in general, the frequency of thesought-for periodic variation in the ambient brightness. An identifiedfrequency may be translated to a code symbol or light-source identifier.Moreover, more than one light source or code symbol may be identified ina single digital image or other series of ambient light measurements byobserving multiple peaks in an FFT.

However, complications arise from the use of rolling-shutter digitalimaging to sample ambient light containing periodic brightnessvariations. As shall be made clearer below, the pixels composing suchimages may be exposed for various lengths of time, as 1/30 second, 1/40second, 1/60 second, or 1/120 second. If the time of pixel exposure isexactly or approximately equal to an integer multiple of the period(peak-to-peak duration) of the periodic brightness variation (signal),the signal may be rendered undetectable. Because cameras mayautonomously set their exposures over a wide range, it may be difficultor impractical to set a number of distinguishable light-source IDfrequencies such that the ID signals always remain detectable regardlessof camera self-setting of exposure time.

A brief review of the rolling shutter exposure process will clarify therelevant relationships between signal frequency and exposure time. Atypical CMOS image sensor consists of an M by N rectangular array of CCDsensors which may be considered as a series of M adjacent rows, each Npixels long. In a rolling shutter exposure, one entire row of the arrayis “exposed” simultaneously—that is, all CCD sensors in that row aresimultaneously electronically switched to a state in which light-inducedcharge is collected in each sensor. Exposure of the row continues forsome fixed exposure or integration time T_(I) (e.g., 1/40 sec) set bythe mobile device user or automatically by the mobile device itself theexposure time. Shortly (i.e., T_(S) seconds) after exposure of the jthrow begins, exposure of the (j+1)th row begins. (Numberingconventionally begins with 0, so row number is j=0, 1, . . . M−1.)Typically, T_(I) is much longer than T_(S), so a number of row exposureswill be commenced during the time it takes to expose a single row. Thetotal time required to expose the whole array of M rows is approximately(M×T_(S))+T_(I). After all rows have been fully exposed (or, in somecases, beginning as soon as the first row is fully exposed) a readoutwave sweeps through the sensor array, harvesting accumulated charge fromeach CCD pixel and assigning a digital numerical value to each CCDcharge magnitude. Each array row is subject to the same exposureinterval T_(I), but begins and ends its exposure at a different time.

These relationships are clarified in FIG. 55A and FIG. 55B. FIG. 55A isa plot of an illustrative sinusoidal brightness signal s(t) 5500 emittedby a light source. The vertical axis 5502 corresponds to brightness andthe horizontal axis 5504 corresponds to time. The signal s(t) 5500 hasan added DC bias comparable to that added to the square-wave signalsdepicted in FIGS. 19A-C; that is, even during the dimmest part of itscycle, s(t) 5500 is nonzero (the light is not completely off). FIG. 55Aalso indicates the time window 5506 of a rolling-shutter row exposure.It is here assumed that acquisition of the rolling shutter frame beginsat time t=0, so row j begins exposure at t=jT_(S) (j=0, 1, 2, . . . M−1)and ends T_(I) seconds later at t=jT_(S)+T_(I). All pixels of row j areexposed simultaneously during this time interval.

FIG. 55B depicts the same set of relationships for exposure of the nextrow of the array, the (j+1)th row. The exposure interval 5506 for the(j+1)th row begins T_(S) seconds after exposure of the jth row, att=(j+1)T_(S), and ends T_(I) seconds later, at t=(j+1)T_(S)+T_(I). Thus,the rolling-shutter digital image is acquired as a staggered oroverlapping series of row exposures.

Because a scene image is usually projected by the camera lens upon theCMOS array, each pixel in each row will typically accumulate a differentamount of charge than its neighbors during its exposure interval T_(I).However, because the brightness signal s(t) is part of overall sceneillumination, it will tend to contribute approximately the same amountof charge to every pixel in a given row (i.e., it illuminates the sensormore or less uniformly). The contribution of s(t) to the chargeaccumulated by each row (as opposed to the pixels within each row) willtend to differ because the interval over which each row integrates s(t)begins and ends at a different time. The result is the characteristicstriping contributed to a rolling-shutter image by a DPR modulated lightsource as described hereinabove.

No striping will appear in the detected image, however, under certainconditions. As will be clear to persons familiar with the art of signalanalysis, when the exposure interval T_(I) is equal to the period T_(S)of s(t) (indicated in FIG. 55A) the contribution of s(t) to the exposureof any row will be equal to the contribution of s(t) to the exposure ofany other row. That is, the integral of s(t) over any interval of lengthT_(S) is equal to the integral of s(t) over any other interval of lengthT_(S), regardless of when the intervals begin. It follows that allintegrations over integer multiples of T_(S) are also equal.

More formally, if C_(j) is the magnitude of the charge accumulated bythe pixels in the jth row as a result of exposure to the s(t) componentof the light impinging on the image sensor, where s(t) is any periodicsignal, not necessarily a sinusoid, then in general

$C_{j} \propto {\int\limits_{{jT}_{S}}^{{jT}_{S} + T_{I}}{{s(t)}d\; t}}$

But if T_(I)=wT_(S), with w an integer, then C_(j)=C_(k)=C for all j,k<M−1. That is, all row exposures to s(t) will be equal, there will beno striping, and s(t) will go undetected. An illustration of this effectis depicted in FIG. 56A. The horizontal axis 5600 is the row index j,and the vertical axis 5602 is the integrated pixel charge magnitudeC_(j) (here presumed identical for all pixels in row j). The plottedcurve 5604 consists of C_(j) values over an arbitrary range of j. TheC_(j) values, being discrete numbers, are indicated by dots, but acontinuous line has been added to curve 5600 for visual clarity. Ineffect, curve 5604 is a slice across the rolling shutter image (ignoringnonuniform background features), perpendicular to its rows and hence topotential DPR striping. In FIG. 56A, T_(I)=wT_(S) with w a positiveinteger, so all C_(j) values are equal, no striping is visible, and s(t)goes undetected.

In FIG. 56B, T_(I) is approximately equal to wT_(S), with w a positiveinteger. In other words, the exposure interval T_(I) is close to theperiod T_(S) (which is indicated in FIG. 56B) of the periodic signals(t). The result is that not all C_(j) values making up the samplesignal 5606 are equal, so s(t) is in principle detectable in the C_(j)values; however, the amplitude A(T_(I)) of the oscillation in the samplesignal 5606 is small. (Here, the notation “A(T_(I))” signifies thatamplitude A is a function of exposure interval T_(I). In FIG. 56A,A(T_(I))=0 and is not indicated.) Thus, when T_(I) is approximatelyequal to wT_(S), with w a positive integer, s(t) will be more difficultto detect in the presence of noise.

In FIG. 56C, T_(I) differs significantly from wT_(S). Thus, differentrows of the image array accumulate distinctly different quantities ofcharge during their exposure to s(t), and s(t) is robustly detectable inthe sample signal 5608. It will be apparent to persons familiar with theart of signal analysis that the amplitude A(T_(I)) of the oscillation inthe sample signal 5606 will be at a maximum if T_(I)=(w+½)T_(S), where wis any integer from 0 on up. However, A(T_(I)) need not be at a maximumin order for s(t) to be detected: A(T_(I)) need only be large enough,relative to noise also present in a digital image, to enablesufficiently robust detection of s(t).

In sum, FIGS. 56A-C demonstrate how three different values of A(T_(I))corresponding to three values of T_(I) for an illustrative sinusoidalDC-offset brightness signal s(t): i.e., zero A(T_(I)) for FIG. 56A,small A(T_(I)) for FIG. 56B, and large A(T_(I)) for FIG. 56C.

FIG. 57A is a plot showing values of A(T_(I)) corresponding to a rangeof values of T_(I) for an illustrative sinusoidal DC-offset brightnesssignal s(t) (not shown). In FIG. 57A, the horizontal axis 5700corresponds to T_(I) (with ten evenly-spaced values of T_(I) labeled ininverse seconds according to the convention for naming camera exposuretimes; note, horizontal axis 5700 orders exposure times from longer atthe left to shorter at the right). The vertical axis 5702 corresponds tothe magnitude of the A(T_(I)) curve 5704. The period T_(S) of thesimulated signal s(t) sampled for FIG. 57A is 1/675 second; therefore,per the foregoing discussion, A(T_(I)) should equal zero—that is, s(t)should be undetectable—at exposure times that are integer multiples of1/675 second, i.e., at T_(I)=wT_(S) with w a positive integer. This isindeed the case, as shown in FIG. 57A (although due to coarseness in therepresentation, the A(T_(I)) curve 5704 does not always go exactly tozero). For example, A(T_(I)) should equal zero for w=8, a positiveinteger, in which case T_(I)=8T_(S)= 8/675 second=1/84.375 second. Thatis, there should be a zero in the A(T_(I)) curve 5704 at 1/84.375second. The zero is indeed there, as indicated by dashed circle 5706.

Moreover, per the foregoing discussion there should be maxima in theA(T_(I)) curve at T_(I)=(w+½)T_(S), where w is any integer from 0 on up.For example, for w=8, T_(I)=(8+½)/675=1/79.4 seconds. The maximum isindeed there, as indicated by dashed circle 5708.

Typically, a mobile device chooses its camera exposure automaticallyfrom a small number of available presets, seeking to maximize imagecontrast. In FIG. 57A, vertical lines 5710, 5712, 5714, and 5716 markcommon exposure times (T_(I) values): 1/30 sec (line 5710), 1/40 sec(line 5712), 1/60 sec (line 5714), and 1/120 sec (line 5716). Darkarrows point to the values of A(T_(I)) corresponding to these fourexposure times. As is apparent, A(T_(I)) is relatively large forexposures of 1/30 sec, 1/60 sec, and 1/120 sec for this s(t) with period1/675 sec, but relatively low for an exposure time of 1/40 sec. Thus, ifa mobile device happens to self-set its exposure time to 1/40 second,the signature of s(t) in the A(T_(I)) curve 5704 may be too weak todetect, particularly in the presence of noise.

FIG. 57B and FIG. 57C illustrate that this problem is not generallyresolvable by changing T_(S). In FIG. 57B, the simulated s(t) (notshown) has T_(S) equal to 1/644 second, and the A(T_(I)) curve 5718 hasshifted leftward, but A(T_(I)) is still low for T_(I)= 1/40 sec. In FIG.57C, the simulated s(t) has T_(S) equal to 1/704 sec, and the A(T_(I))curve 5720 has shifted rightward, and A(T_(I)) is now low for T_(I)=1/120 sec.

In various embodiments, the problem explicated by FIGS. 55A-B, 56A-C,and 57A-B may be mitigated by making s(t) a nonperiodic signal. Thebrightness signal s(t) may be made nonperiodic, yet remain detectableusing the FFT peak detection approach described hereinabove, by“sweeping” its frequency in a relatively narrow range. For a signalperiod T seconds, the frequency of the signal is by definition f=1/T(units of Hertz [Hz], cycles per second). One method of frequencysweeping is to broadcast a repeating or randomly ordered series ofsinusoidal brightness signals of the form s(t)=sin(ω_(i)t), whereω_(i)=2π/T_(S(i)) (in radians per second), the ith of a set of F signalfrequencies. The F frequencies 1/T_(S (c)) are preferably relativelyclosely spaced (evenly or otherwise) around a center frequency1/T_(S(c)) that may or may not itself be one of the F frequencies. Forexample, for a center frequency of 675 Hz, the brightness signal may beswept by cycling through F=8 frequencies spaced at intervals of 15 Hzabove and below the center frequency. Thus, signals are broadcast fromthe light source at 615 Hz, 630 Hz, 645 Hz, 660 Hz, 690 Hz, 705 Hz, 720Hz, and 735 Hz. The cycle is repeated every 2 seconds, so each signal isbroadcast for 2/8=¼ seconds. Other values of F and other cycle repeatperiods may be employed. FIG. 57D shows the A(T_(I)) curve 5722 whenthis frequency sweeping scheme is implemented for sampling by the samerolling-shutter method posited for FIGS. 57A-C. Although the A(T_(I))curve 5722 varies in magnitude, it nowhere approaches zero and thevalues of A(T_(I)) at the four exemplary exposures (marked by boldarrows) are similar.

The camera will detect the brightness signal s(t) in image framesexposed significantly or entirely when a frequency is being broadcastthat is compatible with the exposure that happens to have been chosen bythe camera. Thus, some frames will reveal s(t) strongly and others willnot. By averaging multiple frames, detection of s(t) is enabled.

Other methods of frequency sweeping are also contemplated for variousembodiments. For example, the frequency of a quasi-sinusoidal signalcould be continuously varied. In this approach, instead of modulating alight source to broadcast a sinusoidal brightness signal such ass(t)=sin(ωt), where ω=2π/T_(S), the light source is modulated totransmit a brightness signal s(t)=sin((ω+R sin(φt))t) where R is aconstant that sets the width of the sweep range and φ is the frequency(in radians per second) of the sweep cycle applied to the centerfrequency ω. Or, sweeping could occur in a randomized fashioned (randomrange of sweep, random speed of sweep, randomized jumping to discretefrequencies within the sweep range, etc.). It will be clear to personsfamiliar with the art of signal processing that these and many otherschemes for frequency sweeping are conceivable, have various advantagesand disadvantages, and could be implemented in various embodiments ofthe invention without undue experimentation. It will also be clear thatsome methods of frequency sweeping may be applied to any periodicsignal, not only the sinusoidal signals illustrated herein. All suchembodiments are contemplated and within the scope of the invention.

Modulation of Light Signals to Transmit Positional Information

In various embodiments, as discussed hereinabove, modulated brightnessmay encode information distinctively identifying light sources. Thisidentifying information may be used to look up the physical location ofthe light source in a database. In various other embodiments, modulationof light from light sources may directly encode location and otherinformation. In one illustrative embodiment, this is accomplished asfollows: (1) An x-y coordinate system is defined over a rectangularphysical space (planar area) that is large enough to cover or include agiven working space, e.g., a retail store interior, that is to be servedby a light-based positioning system. The working space may or may not beitself rectangular but is covered by the coordinate system. (2) Thebrightness of each of one or more light sources in the physical space ismodulated by a signal that includes or consists essentially of two ormore superimposed sinusoids. In one example, a frequency of one of thesinusoids has a defined relationship to the x coordinate and a frequencyof another of the sinusoids has a defined relationship to the ycoordinate. The frequency of one of the two sinusoids broadcast by thesource, for example, is set, during an initial commissioning orprogramming process, to be proportional to the x coordinate of the lightsource in the physical space. Similarly, the frequency of the othersinusoid, for example, is set to be proportional to the y coordinate ofthe light source. The proportionality may be either direct (e.g., aconstant ratio such as f_(x):x, f_(y):y) or linear (e.g., f_(x)=mx+c,f_(y)=my+c, where m is a slope defined by a ratio and c is a constant).If no light sources in the working space are co-located, then thex-coordinate frequency and y-coordinate frequency associated with agiven light will uniquely specify the light's location (and thusidentity). (3) A mobile device detects the frequencies of thex-coordinate and y-coordinate sinusoidal signals (by, e.g., detectingpeaks in an FFT of camera or light-sensor data acquired by the mobiledevice). (4) Software in the mobile device, or in a computer with whichthe mobile device is in communication, matches the measured sinusoidfrequencies to a light's physical x-y coordinates in a database orlookup table, establishing the physical location of the mobile device.

FIG. 58A is a schematic overhead view of the location of light source5800 in a rectangular physical space 5802, according to an illustrativeembodiment of the invention. Light 5800 is part of a light-basedpositioning system that may contain a multiplicity of similar lightsources. Not all of the rectangular space 5802 need be navigable (e.g.,some portions of space 5802 may lie within a building, and otherportions may lie outside the building), but in general the rectangularspace 5802 will be large enough to include a working space served by thelight-based positioning system. As indicated in FIG. 58A, the locationof light source 5800 is specified by x and y coordinates of a Cartesiancoordinate system aligned with the rectangular space 5802. The x and ycoordinate axes are defined so that both the x and y dimensions of thespace 5802 correspond to a numerical range of [0,1], with the origin or(0,0) point of the x-y coordinate system coinciding with one corner ofthe space 5802. The physical distance of a point in the space 5802 inthe x dimension, as measured from the origin, is found by multiplyingthe point's x coordinate on [0,1] by the physical width of the space5802 in the x dimension. The physical y coordinate of a point in thespace 5802 is similarly obtained. For example, in the illustrative casedepicted in FIG. 58A, the light source 5800 is located at (x, y)=(0.5,0.75). If the space 5802 is 100 meters wide in the x dimension and 50meters wide in the y dimension, the physical coordinates of the lightsource 5801 are therefore (x_(physical), y_(physical))=(0.5×100 m,0.75×75 m)=(50 m, 37.5 m).

Various embodiments encode the (x, y) coordinates of the light source5800 in the light emitted by the light 5800. In one embodiment, thebrightness of the light 5800 is modulated simultaneously by twosinusoids, S_(x) and S_(y), that may have different frequencies. TheS_(x) sinusoid encodes the x coordinate of the light 5800 and the S_(y)sinusoid encodes the y coordinate. The frequency of S_(x) is hereintermed f_(x), and the frequency of S_(y) is termed f_(y). The sinusoidsmay be detected and their frequencies measured by a mobile or otherdevice using various methods, including peak-detection of an FFT of aseries of brightness observations made by a digital camera or any othersuitable optical sensor.

A device that has detected two brightness sinusoids 5804, 5806 in lightbroadcast by source 5800 in FIG. 58A must be able to distinguish whichis S_(x) and which is S_(y) in order to correctly infer the location ofsource 5800. FIG. 58B depicts the spectral character of S_(x) 5804 andS_(y) 5806 according to an illustrative embodiment that enables suchdistinction. In the amplitude-coding scheme depicted in FIG. 58B, S_(x)and S_(y) broadcast by source 5800 at (x, y) are assigned frequenciesf_(x) and f_(y) between a lower limit f_(start) and an upper limitf_(stop). In particular, f_(x) is offset from f_(start) f by an amountproportional to x (i.e., f_(x)=f_(start)+x(f_(stop)−f_(start)) and S_(y)is assigned a frequency f_(y) by the same method (i.e.,f_(y)=f_(start)+y(f_(stop)−f_(start)). A receiving device maydistinguish S_(x) from S_(y) by the fact that S_(x) has a greateramplitude than S_(y). A drawback of this method is that it deliberatelymakes S_(y) more difficult to detect than S_(x).

FIG. 58C depicts the spectral character of S_(x) 5804 and S_(y) 5806according to another illustrative embodiment. In the frequency-codingscheme depicted in FIG. 58C, S_(x) 5804 is assigned a frequency in onefrequency band (i.e., between a lower limit f_(x-start) and an upperlimit f_(x-stop)) and S_(y) 5806 is assigned a frequency in a secondfrequency band (i.e., between a lower limit f_(y-start) and an upperlimit f_(y-stop)). To assure that the two signals S_(x) 5804 and S_(y)5806 are always distinguishable by a detecting device, the upperfrequency of one band is separated from the lower frequency of the otherband by a buffer difference Δbuff. To encode the x-y coordinates of asource 5800 (FIG. 58A), f_(x) is offset from f_(x-start) by an amountproportional to x (i.e., f_(x)=f_(x-start)+x(f_(x-stop)−f_(x-start)))and f_(y) is offset from f_(y-start) by an amount proportional to y(i.e., f_(y)=f_(y-start)+y(f_(y-stop)−f_(y-start))). The two sinusoidsS_(x) 5804 and S_(y) 5806 may, in this illustrative embodiment, be ofapproximately equal magnitude. In various embodiments, the two frequencybands employed may or may not be of equal width; also, the coding schememay be modified to employ more than two bands, thus conveying additionalinformation or spreading the x and y coordinate information overmultiple, non-contiguous bands.

To decode the broadcast information, a detecting device detects the twosinusoids and infers from the positions of their frequencies withintheir non-overlapping bands the physical coordinates of the source 5800in FIG. 58A.

The scheme depicted in FIG. 58C has the drawback of employing twononoverlapping frequency bands, even though in communications systems itis in general desirable to minimize the use of bandwidth. In variousother embodiments, a single frequency band (shared x-y band) is employedto broadcast a source's x and y coordinates. Coding schemes of this typeare herein termed “XY compression” schemes. FIG. 59 depicts the meaningof several terms pertaining to an illustrative XY compression scheme.The shared x-y band begins at a lower limit f_(start) f and ends at anupper limit f_(stop). Three sinusoids are broadcast simultaneously, i.e.(1) S_(x) (not depicted), having frequency f_(x), (2) S_(y-plus) (notdepicted), having frequency f_(y-plus), and (3) S_(y-minus) (notdepicted), having frequency f_(y-minus). All three sinusoids may haveapproximately the same amplitude (e.g., the maximum amplitude that maybe feasibly broadcast). To assure that all sinusoids may bedistinguished by a detecting device, the frequency difference betweenany two sinusoids is always equal to or greater than a buffer differenceΔbuff. As depicted in FIG. 60A, the ranges permitted to the frequenciesof the three sinusoids S_(x), S_(y-plus), and S_(y-minus) are asfollows: For f_(x), the start frequency (lower limit) isf_(x-start)=f_(start)+Δbuff and the end frequency (upper limit) isf_(x-stop)=f_(stop)−Δbuff. For f_(y-plus), the start frequency isf_(y-plus-start)=f_(start)+2Δbuff and the end frequencyf_(y-plus-stop)=f_(stop). For f_(y-minus), the start frequency isf_(y-minus-start)=f_(start) and the end frequency isf_(y-minus-stop)=f_(stop)−2Δbuff.

FIG. 60 depicts the encoding of the same illustrative light-sourcecoordinates depicted in FIG. 58A using a version of the scheme partlydepicted in FIG. 59. Three sinusoids are broadcast simultaneouslybetween f_(start) and f_(stop), i.e. (1) S_(x) 5804, having frequencyf_(x), (2) S_(y-plus) 6000, having frequency f_(y-plus), and (3)S_(y-minus) 6002, having frequency f_(y-minus). A minimum difference ofΔbuff is maintained between adjacent frequencies. The x coordinate isencoded by the position of f_(x) in the shared band and the y coordinateis encoded jointly by the positions of f_(y-minus) and f_(y-plus) in theshared band. First, f_(x) is determined as a proportional fraction ofthe x-frequency range: f_(x)=f_(x-start)+x(f_(x-stop)−f_(x-start)).Second, f_(y-minus) is set equal to f_(start). Third, f_(y-plus) is setto a proportional fraction of the f_(y-plus) frequency range:f_(y-plus)=f_(y-plus-start)+y(f_(y-plus-stop)−f_(y-plus-start)).However, if this value of f_(y-plus) is less than f_(x)+Δbuff, thenf_(y-minus) and f_(y-plus) are both right-shifted, as explained below,to assure that (a) f_(x) remains between f_(y-minus) and f_(y-plus)and/or that (b) a difference of Δbuff is preserved between adjacentfrequencies.

In an illustrative case, f_(start)=200 Hz, f_(stop)=800 Hz, Δbuff=50 Hz,and the x-y coordinate pair to be broadcast is (0.5, 0.75). Therefore,f _(x-start) =f _(start)+Δbuff=200+50 Hz=250 Hzf _(x-stop) =f _(stop)−Δbuff=800−50=750 Hzf _(y-plus-start) =f _(start)+2Δbuff=200+2(50) Hz=300 Hzf_(y-plus-stop)=f_(stop)=800 Hzf_(y-minus-start)=f_(start)=200 Hzf_(y-minus-stop)=f_(stop)−2Δbuff=800−2(50) Hz=700 HzAlso,f _(x) =f _(x-start) +x(f _(stop) −f _(start)−2Δbuff)=f _(x-start) +x(f_(x-stop) −f _(x-start))=250+0.5(750−250)=500 Hzf_(y-minus)=f_(start)=200 Hzf _(y-plus) =f _(y-plus-start) +y(f _(y-plus-stop) −f_(y-plus-start))=300+0.75(800−300)=675 Hz.These illustrative values are depicted in FIG. 60.

In another illustrative case, the x-y coordinate pair to be broadcast is(0.5, 0.25). This case is schematically depicted in FIG. 61A. In thiscase,f _(x) =f _(x-start) +x(f _(stop) −f _(start)−2Δbuff)=f _(x-start) +x(f_(x-stop) −f _(x-start))=250+0.5(750−250)=500 HzHowever, here the default value for f_(y-plus) is less than f_(x)+Δbuff(=550 Hz):f _(y-plus-start) +y(f _(y-plus-stop) −f_(y-plus-start))=300+(0.25)(800−300)=425 Hz<550 HzTherefore, f_(y-minus) and f_(y-plus) are assigned right-shifted valuesaccording to the following rules:f _(y-plus) =f _(x)+Δbuff=500+50 Hz=550 Hzf _(y-minus) =f _(y-minus-start) +f _(x)+Δbuff−(f _(y-plus-start) +y(f_(y-plus-stop) −f _(y-plus-start)))=200+500+50−425 Hz=325 HzThese illustrative values are depicted in FIG. 61B.

FIG. 62 depicts an example of a process performed by a mobile device toidentify a location of a lighting device within a space. In step 6201, alighting device emits visible artificial light within a space. In oneexample, the emitted visible artificial light includes modulatedintensities representing at least two frequencies corresponding to atleast two sinusoids, such as described above in relation to FIGS.58A-61B. An image sensor of a mobile device, in step S6202, captures oneor more images including the emitted visible light. The mobile deviceperforms frequency analysis in step 6203. As part of the frequencyanalysis, the mobile device obtains the at least two frequencies.

In step 6204, the mobile device infers an x coordinate corresponding tothe location of the lighting device. In step 6205, the mobile deviceinfers a y coordinate corresponding to the location of the lightingdevice. The x and y coordinates, for example, represent a physicallocation of the lighting device within a space, as described above inrelation to FIGS. 58A and 61A. Based on the inferred x,y coordinates,the location of the lighting device is determined in step 6206 and theprocess ends.

In one example, corresponding to the shared-band, 2-amplitude codingdescribed above in reference to FIG. 58B, the mobile device identifiesthe obtained frequency having the greater amplitude as representing thex coordinate and the obtained frequency having the lesser amplitude asrepresenting the y coordinate. The mobile device, in this example, alsodetermines a lower limit frequency and an upper limit frequency. In thisexample, the value of the x coordinate is equal to the differencebetween the obtained frequency having the greater amplitude and thelower limit frequency divided by the difference between the upper limitfrequency and the lower limit frequency. Similarly, the value of the ycoordinate is equal to the difference between the obtained frequencyhaving the lesser amplitude and the lower limit frequency divided by thedifference between the upper limit frequency and the lower limitfrequency.

In a different example, corresponding to the split-band coding describedabove in relation to FIG. 58C, the mobile device identifies a first ofthe two obtained frequencies between a first lower limit frequency and afirst upper limit frequency as representing the x coordinate. The mobiledevice, in this different example, also identifies a second of the twoobtained frequencies between a second lower limit frequency and a secondupper limit frequency as representing the y coordinate. The mobiledevice further determines a buffer difference between the two frequencyranges. In this different example, the value of the x coordinate isequal to the difference between the first obtained frequency and thefirst lower limit frequency divided by the difference between the firstupper limit frequency and the first lower limit frequency. Similarly,the value of the y coordinate is equal to the difference between thesecond obtained frequency and the second lower limit frequency dividedby the difference between the second upper limit frequency and thesecond lower limit frequency.

In yet another example, corresponding to the shared-band,three-frequency coding described above in relation to FIG. 59, themobile device obtains three frequencies modulated within the emittedlight. The mobile device first determines a lower limit frequency, anupper limit frequency and a buffer difference. The mobile device, inthis yet another example, identifies a first of the three obtainedfrequencies between a first additional lower limit frequency and a firstadditional upper limit frequency as representing the x coordinate. Thefirst additional lower limit frequency is equal to the lower limitfrequency plus the buffer difference and the first additional lowerlimit frequency is equal to the upper limit frequency minus the bufferdifference. The mobile device then identifies a second of the threeobtained frequencies between a second additional lower limit frequencyand a second additional upper limit frequency. The second additionallower limit frequency is equal to the lower limit frequency and thesecond additional lower limit frequency is equal to the upper limitfrequency minus twice the buffer difference. The mobile device alsoidentifies a third of the three obtained frequencies between a thirdadditional lower limit frequency and a third additional upper limitfrequency as representing the y coordinate. The third additional lowerlimit frequency is equal to the lower limit frequency plus twice thebuffer difference and the third additional upper limit frequency equalto the upper limit frequency. That is, the frequency corresponding tothe y frequency is always the highest obtained frequency and thefrequency corresponding to the x frequency is always between the othertwo obtained frequencies.

In this yet another example, the value of the x coordinate is equal tothe difference between the first obtained frequency and the firstadditional lower limit frequency divided by the difference between thefirst additional upper limit frequency and the first additional lowerlimit frequency. Similarly, the value of the y coordinate is equal tothe difference between the third obtained frequency and the thirdadditional lower limit frequency divided by the difference between thethird additional upper limit frequency and the third additional lowerlimit frequency.

It will be clear to a person of ordinary skill in the science ofcommunications and signaling that the rules demonstrated with referenceto FIGS. 59-61B will (a) always produce values of the frequencies f_(x),f_(y-plus), and f_(y-minus) that (a) always place f_(x) betweenf_(y-minus) and f_(y-plus), (b) never allow two of the frequencies to becloser than Δbuff, and (c) uniquely and simultaneously map any xcoordinate on [0, 1] and any y coordinate on [0,1] to the sharedfrequency band, enabling a receiving device to infer the transmittedcoordinates. It will also be clear that various details of theseillustrative schemes may be varied without the introduction ofmeaningful inventive novelty; for example, non-sinusoidal modulations ofbrightness could be employed, the roles of x and y could be reversed,different algebraic rules for setting the three frequencies could beemployed, and so forth. All such variations are contemplated and withinthe scope of the invention.

Frequency Sweeping to Mitigate Destructive Interference

A limitation of the illustrative XY compression scheme described withreference to FIGS. 59-61B is that two or more light sources employingthe same frequency range for the broadcast of x and y coordinateinformation may illuminate a given area using sinusoids having identicalfrequencies: e.g., two light sources may have different y coordinatesbut the same x coordinate. Although such frequency matching will notintroduce informational ambiguity (e.g., if two light sources detectableby a mobile device have the same x coordinate then it does not matter ifone source's S_(x) signal, or the other's, or both are detected by themobile device), but destructive interference by out-of-phase sinusoidsof identical frequency may occur. Such destructive interference may makethe sinusoids undetectable at various points in the illuminated physicalspace, producing “dead spots.” Therefore, in various embodiments of theinvention featuring multiple light sources that broadcast locationinformation using the XY compression scheme, destructive interference isavoided by causing each light source to sweep its broadcast sinusoidfrequencies repetitively over a randomly or pseudorandomly varied sweepperiod. For example, a light source broadcasting a sinusoid having anominal frequency of 310 Hz may sweep the actual frequency of thesinusoid from 305 to 315 Hz over a period of 2.1 minutes, while anotherlight source, also broadcasting a sinusoid having a nominal frequency of310 Hz, may sweep the actual broadcast frequency of its sinusoid from305 to 315 Hz over a period of 2.0 minutes. Sweep periods may berandomly and permanently assigned to individual light sources, orindividual light sources may vary their own sweep periods in a randomfashion; sweeping may occur either continuously or in discrete steps offrequency change. In either case, the result is that any interferencepatterns occurring in the illuminated space are impermanent and thereare no fixed “blind spots.” For example, any destructive interferencefrom nominal 310 Hz signals broadcast by two light sources cannot belocked in, but will be fleeting.

Additionally or alternatively to modulation of the brightness of lightsources in order to convey identifier and/or location information,modulation of the frequency of light sources to convey such informationis contemplated and within the scope of the invention. Perceptiblefrequency modulation (color modulation) would likely be irksome to usersof a beacon-illuminated space, but color modulation will not beperceptible if (a) the modulation is over a sufficiently narrowfrequency range, (b) the modulation is sufficiently rapid compared tohuman persistence of vision, and/or (c) the light being modulated is notvisible, e.g., the light is infrared light. Color modulation is readilydetectable by a range of devices, including digital cameras such asthose built into many mobile devices.

Moreover, devices exist for modulating the polarization of light (e.g.,electronically controlled Faraday rotators), and such devices may beminiaturized (using, e.g., micro-electromechanical systems [MEMS]techniques). Polarity-modulated light may thus also be broadcast by alight source. Not all the light broadcast by a light source need be somodulated. Changes in polarization may both be detected by a variety ofmeans, as will be clear to persons familiar with the science of physicaloptics. Thus, information may be broadcast by modulating the brightnessand/or frequency and/or polarization of some or all of the light from alight source in either a fixed or a time-varying fashion. Indeed,modulation of two or more of brightness, frequency, and polarization maybe performed simultaneously, effectively conveying light-identifyinginformation, x-y coordinate information, and other information along twoor more parallel channels and so increasing the effective bandwidth ofthe source, which is advantageous. Mobile devices are not yet typicallyequipped with sensors enabling the detection of infrared frequencymodulation and/or polarization modulation, but mobile devices could beso equipped. All techniques discussed herein that employ brightnessmodulation as a means of broadcasting information from light sources, orthat are contemplated and within the scope of the invention though notexplicitly discussed, are also contemplated and within the scope of theinvention insofar as these techniques may also employ forms of lightmodulation other than, or in addition to, brightness modulation. Forexample, position determination by a mobile device may entailidentifying nearby light sources by distinctive sinusoidal variations incolor or polarization rather than, or in addition to, distinctivesinusoidal variations in brightness.

The techniques and methods disclosed herein for use in light-basedpositioning systems can be used with a variety of camera equipped mobileor stationary devices, such as mobile phones, tablet computers,netbooks, laptops, desktops, wearable computers, computer-enhancedeyeglasses, or custom-designed hardware.

Having described one embodiment of the invention, it will be apparent tothose of ordinary skill in the art that other embodiments incorporatingthe concepts disclosed herein may be used without departing from thespirit and scope of the invention. The described embodiments are to beconsidered in all respects as only illustrative and not restrictive.

What is claimed is:
 1. A lighting device, comprising: a light source; amodulator coupled to the light source; and a processor coupled to themodulator and configured to: control the light source to emit visiblelight for general illumination within a space; and control the modulatorto: modulate the intensity of visible light emitted by the light sourcebased on a signal comprising at least two superimposed sinusoids and inaccordance with at least two frequencies of the at least twosuperimposed sinusoids such that the at least two superimposed sinusoidsare simultaneously broadcast; sweep across the respective frequency of afirst of the at least two superimposed sinusoids during a respectivesweep cycle that spans a timeframe of a respective sweep period by:varying the frequency of the first of the at least two superimposedsinusoids, between a number of varied frequencies and within a frequencymodulation range, during each of a plurality of sub-cycles of the sweepcycle, each sub-cycle corresponding to a respective fraction of thetimeframe; and maintaining each respective varied frequency of the firstof the at least two superimposed sinusoids during a respective sub-cyclefor the respective fraction of timeframe, each fraction of the timeframecorresponding to the respective sub-cycle such that the collection offractions of the timeframe for the respective number of variedfrequencies of the respective sweep cycle equals the timeframe of therespective sweep period.
 2. The lighting device of claim 1, wherein: aphysical location of the lighting device within the space corresponds toa location defined by an x,y coordinate system over a planar areaencompassing the space; a frequency of a first of the at least twosuperimposed sinusoids has a defined relationship to a value of an xcoordinate of the physical location of the lighting device in the x,ycoordinate system; and a frequency of a second of the at least twosuperimposed sinusoids has a defined relationship to a value of a ycoordinate of the physical location of the lighting device in the x,ycoordinate system.
 3. The lighting device of claim 2, wherein: afrequency of a third of the at least two superimposed sinusoids has adefined relationship to the y coordinate of the physical location of thelighting device in the x,y coordinate system; and the processor isfurther configured to: limit a frequency range of the first, second andthird superimposed sinusoids by a lower limit frequency (f_(start)) andan upper limit frequency (f_(stop)); further limit the frequency rangeof the first superimposed sinusoid (f_(x)) by: a first additional lowerlimit frequency (f_(x-start)) equal to the lower limit frequency plus abuffer difference (f_(x-start)=f_(start)+Δbuff); and a first additionalupper limit frequency (f_(x-stop)) equal to the upper limit frequencyminus the buffer difference (f_(x-stop)=f_(stop)−Δbuff); further limitthe frequency range of the second superimposed sinusoid (f_(y-minus))by: a second additional lower limit frequency (f_(y-minus-start)) equalto the lower limit frequency (f_(y-minus-start)=f_(start)); and a secondadditional lower limit frequency (f_(y-minus-stop)) equal to the upperlimit frequency minus twice the buffer difference(f_(y-minus-stop)=f_(stop)−2Δbuff); further limit the frequency range ofthe third superimposed sinusoid (f_(y-plus)) by: a third additionallower limit frequency (f_(y-plus-start)) equal to the lower limitfrequency plus twice the buffer difference(f_(y-plus-start)=f_(start)+2Δbuff); and a third additional upper limitfrequency (f_(y-plus-stop)) equal to the upper limit frequency(f_(y-plus-stop)=f_(stop)); set the frequency of the first superimposedsinusoid (f_(x)) equal to the first additional lower limit frequencyplus the value of the x coordinate multiplied by the difference betweenthe first additional upper limit frequency and the first additionallower limit frequency (f_(x)=f_(x-start)+x(f_(x-stop)−f_(x-start))); setthe frequency of the second superimposed sinusoid (f_(y-minus)) equal tothe lower limit frequency (f_(y-minus)=f_(start)); and set the frequencyof the third superimposed sinusoid (f_(y-plus)) equal to the thirdadditional lower limit frequency plus the value of the y coordinatemultiplied by the difference between the third additional upper limitfrequency and the third additional lower limit frequency(f_(y-plus)=f_(y-plus-start)+y(f_(y-plus-stop)−f_(y-plus-start))). 4.The lighting device of claim 3, wherein the processor is furtherconfigured to: compare the frequency of the first superimposed sinusoid(f_(x)) with the frequency of the third superimposed sinusoid(f_(y-plus)); and upon a determination that the frequency of the thirdsuperimposed sinusoid is less than the frequency of the firstsuperimposed sinusoid plus the buffer difference(f_(y-plus)>f_(x)+Δbuff): set the frequency of the third superimposedsinusoid equal to the frequency of the first superimposed sinusoid plusthe buffer difference (f_(y-plus)=f_(x)+Δbuff); and set the frequency ofthe second superimposed sinusoid equal to the second additional lowerlimit frequency plus the frequency of the first superimposed sinusoidplus the buffer difference minus the sum of the third additional lowerlimit frequency and the value of the y coordinate multiplied by thedifference of the third additional upper limit frequency and the thirdadditional lower limit frequency(f_(y-minus-start)+f_(x)+Δbuff−(f_(y-plus-start)+y(f_(y-plus-stop)−f_(y-plus-start)))).5. The lighting device of claim 1, wherein: a center frequency of thefrequency modulation range corresponds to the frequency of the first ofthe at least two superimposed sinusoids; the respective variedfrequencies during a respective sub-cycle form a sequence; any twoimmediately proximate varied frequencies within the sequence differ by adifference value; and the modulator varies the respective frequencybetween the number of varied frequencies during each respectivesub-cycle such that the frequency of the first of the at least twosuperimposed sinusoids varies across the frequency modulation rangearound the center frequency during each respective sub-cycle.
 6. Thelighting device of claim 5, wherein: the difference value is constantwithin each sub-cycle; and the number of modulation rates is even duringeach sub-cycle, such that an equal number of the rates of modulationoccur above and below the center frequency during each sweep cycle. 7.The lighting device of claim 5, wherein the difference value varieswithin each sub-cycle.
 8. The lighting device of claim 5, wherein thedifference value varies randomly within each sub-cycle.
 9. The lightingdevice of claim 5, wherein: the difference value is constant within eachsub-cycle; and the difference value varies between sub-cycles.
 10. Thelighting device of claim 9, wherein the difference value varies randomlybetween sub-cycles.
 11. The lighting device of claim 1, wherein: theprocessor coupled to the modulator is further configured to control themodulator to sweep across the respective frequency of the first of theat least two superimposed sinusoids during each of a plurality ofsuccessive sweep cycles; and each respective timeframe corresponding toeach sweep cycle varies between the successive sweep cycles.
 12. Thelighting device of claim 1, wherein: the processor coupled to themodulator is further configured to control the modulator to sweep acrossthe respective frequency of the first of the at least two superimposedsinusoids during each of a plurality of successive sweep cycles; andeach respective timeframe corresponding to each sweep cycle variesrandomly between the successive sweep cycles.
 13. The lighting device ofclaim 1, wherein: the processor coupled to the modulator is furtherconfigured to control the modulator to sweep across the respectivefrequency of the first of the at least two superimposed sinusoids duringeach of a plurality of successive sweep cycles; and the frequencymodulation range varies between each sweep cycle.
 14. The lightingdevice of claim 1, wherein: the processor coupled to the modulator isfurther configured to control the modulator to sweep across therespective frequency of the first of the at least two superimposedsinusoids during each of a plurality of successive sweep cycles; and thefrequency modulation range varies randomly between each sweep cycle. 15.The lighting device of claim 1, wherein the processor is furtherconfigured to control the modulator to: vary the frequency of another ofthe at least two superimposed sinusoids, between a second number ofvaried frequencies and within a second frequency modulation range,during each of the plurality of sub-cycles; and maintain each respectivevaried frequency of the other of the at least two superimposed sinusoidsduring a respective sub-cycle for a respective fraction of thetimeframe, each fraction of the timeframe corresponding to therespective sub-cycle such that the collection of fractions of thetimeframe for the respective second number of varied frequencies of therespective sweep cycle equals the timeframe of the respective sweepperiod.
 16. The lighting device of claim 1, wherein the x coordinate isa value between 0 and 1 representing a proportional distance of thelighting device along a physical width of the space and the y coordinateis a value between 0 and 1 representing a proportional distance of thelighting device along a physical length of the space.
 17. A method,comprising steps to: operate an artificial light source to emit visibleartificial light for general illumination within a space; and modulate,based on a signal comprising at least two superimposed sinusoids, theintensity of visible artificial light emitted by the light source tobroadcast a modulated visible light signal including the at least twosuperimposed sinusoids modulated on the emitted visible artificiallight; sweep across the respective frequency of a first of the at leasttwo superimposed sinusoids during a respective sweep cycle that spans atimeframe of a respective sweep period by: varying the frequency of thefirst of the at least two superimposed sinusoids, between a number ofvaried frequencies and within a frequency modulation range, during eachof a plurality of sub-cycles, each sub-cycle corresponding to arespective fraction of the timeframe; and maintaining each respectivevaried frequency of the first of the at least two superimposed sinusoidsduring a respective sub-cycle for the respective fraction of thetimeframe, each fraction of the timeframe corresponding to therespective sub-cycle such that the collection of fractions of thetimeframe for the respective number of varied frequencies of therespective sweep cycle equals the timeframe of the respective sweepperiod.
 18. The method of claim 17, wherein: a physical location of thelighting device within the space corresponds to a location defined by anx,y coordinate system over a planar area encompassing the space; afrequency of a first of the at least two superimposed sinusoids has adefined relationship to a value of an x coordinate of the physicallocation of the lighting device in the x,y coordinate system; and afrequency of a second of the at least two superimposed sinusoids has adefined relationship to a value of a y coordinate of the physicallocation of the lighting device in the x,y coordinate system.
 19. Themethod of claim 18, wherein a frequency of a third of the at least twosuperimposed sinusoids has a defined relationship to the value of the ycoordinate of the physical location of the lighting device in the x,ycoordinate system; and further comprising steps to: limit a frequencyrange of the first, second and third superimposed sinusoids by a lowerlimit frequency (f_(start)) and an upper limit frequency (f_(stop));further limit the frequency range of the first superimposed sinusoid(f_(x)) by: a first additional lower limit frequency (f_(x-start)) equalto the lower limit frequency plus a buffer difference(f_(x-start)=f_(start)+Δbuff); and a first additional upper limitfrequency (f_(x-stop)) equal to the upper limit frequency minus thebuffer difference (f_(x-stop)=f_(stop)−Δbuff); further limit thefrequency range of the second superimposed sinusoid (f_(y-minus)) by: asecond additional lower limit frequency (f_(y-minus-start)) equal to thelower limit frequency (f_(y-minus-start)=f_(start)); and a secondadditional lower limit frequency (f_(y-minus-stop)) equal to the upperlimit frequency minus twice the buffer difference(f_(y-minus-stop)=f_(stop)−2 Δbuff); further limit the frequency rangeof the third superimposed sinusoid (f_(y-plus)) by: a third additionallower limit frequency (f_(y-plus-start)) equal to the lower limitfrequency plus twice the buffer difference(f_(y-plus-start)=f_(start)+2Δbuff); and a third additional upper limitfrequency (f_(y-plus-stop)) equal to the upper limit frequency(f_(y-plus-stop)=f_(stop)); set the frequency of the first superimposedsinusoid (f_(x)) equal to the first additional lower limit frequencyplus the value of the x coordinate multiplied by the difference betweenthe first additional upper limit frequency and the first additionallower limit frequency (f_(x)=f_(x-start)+x(f_(x-stop)−f_(x-start))); setthe frequency of the second superimposed sinusoid (f_(y-minus)) equal tothe lower limit frequency (f_(y-minus)=f_(start)); and set the frequencyof the third superimposed sinusoid (f_(y-plus)) equal to the thirdadditional lower limit frequency plus the value of the y coordinatemultiplied by the difference between the third additional upper limitfrequency and the third additional lower limit frequency(f_(y-plus)=f_(y-plus-start)+y(f_(y-plus-stop)−f_(y-plus-start))). 20.The method of claim 19, further comprising steps to: compare thefrequency of the first superimposed sinusoid (f_(x)) with the frequencyof the third superimposed sinusoid (f_(y-plus)); and upon adetermination that the frequency of the third superimposed sinusoid isless than the frequency of the first superimposed sinusoid plus thebuffer difference (f_(y-plus)<f_(x)+Δbuff): set the frequency of thethird superimposed sinusoid equal to the frequency of the firstsuperimposed sinusoid plus the buffer difference(f_(y-plus)=f_(x)+Δbuff); and set the frequency of the secondsuperimposed sinusoid equal to the second additional lower limitfrequency plus the frequency of the first superimposed sinusoid plus thebuffer difference minus the sum of the third additional lower limitfrequency and the value of the y coordinate multiplied by the differenceof the third additional upper limit frequency and the third additionallower limit frequency(f_(y-minus-start)+f_(x)+Δbuff−(f_(y-plus-start)+y(f_(y-plus-stop)−f_(y-plus-start)))).21. The method of claim 17, wherein: a center frequency of the frequencymodulation range corresponds to a frequency of the signal comprising atleast two superimposed sinusoids; the respective rates of modulationduring a respective sub-cycle form a sequence; any two immediatelyproximate rates of modulation within the sequence differ by a differencevalue; and the respective rates of modulation vary between the number ofmodulation rates during each respective sub-cycle such that a frequencyof the modulated visible light signal varies across the frequencymodulation range around the center frequency during each respectivesub-cycle.
 22. The method of claim 21, wherein: the difference value isconstant within each sub-cycle; and the number of modulation rates iseven during each sub-cycle, such that an equal number of the rates ofmodulation occur above and below the center frequency during eachsub-cycle.
 23. The method of claim 17, further comprising steps to: varythe frequency of another of the at least two superimposed sinusoids,between a second number of varied frequencies and within a secondfrequency modulation range, during each of the plurality of sub-cycles;and maintain each respective varied frequency of the other of the atleast two superimposed sinusoids during a respective sub-cycle for arespective fraction of the timeframe, each fraction of the timeframecorresponding to the respective sub-cycle such that the collection offractions of the timeframe for the respective second number of variedfrequencies of the respective sub-cycle equals the timeframe of therespective sweep period.
 24. The method of claim 17 wherein the xcoordinate is a value between 0 and 1 representing a proportionaldistance of the artificial light source along a physical width of thespace and the y coordinate is a value between 0 and 1 representing aproportional distance of the artificial light source along a physicallength of the space.
 25. A non-transitory tangible computer readablemedium embodying instructions to cause a mobile device to perform themethod of claim 17.